ultrasonography in obstetrics

 GENERAL INTRODUCTION

• The application of ultrasound in obstetrics and gynecology was first described in Europe in 1958 by Donald et al and in America in 1964 by Taylor et al.

• Ultrasound has rapidly replaced X-ray studies in the field of obstetrics and gynecology due to its safety, noninvasiveness, and effectiveness in scanning the abdominal and pelvic regions.

• Ultrasound is considered an ideal scanning procedure for obstetrics and gynecology due to its atraumatic nature and noninvasive approach.

• The ultrasound examination is typically performed with the patient in the supine position.

• A coupling agent, such as mineral oil, is applied to the skin to facilitate the transmission of sound waves.

• Reference points like the umbilicus or the symphysis pubis are used during the examination.

• In obstetric studies, the transrectal plane and puboxiphoid line are often used as reference points, especially in cases of progressing conception and increasing fetal size.

• Transverse, longitudinal, and oblique scans with varying angulations are performed to obtain a diagnostic set of data that can be compared over time.

• B-mode or gray-scale two-dimensional ultrasound, combined with real-time scanning, provides the most diagnostic information in obstetrics.

• In specific areas like the fetal head, the usage of A-mode ultrasound adds more information and improves accuracy.

PATIENT HISTORY

• The examiner's first step in using ultrasound is to gather a proper history from the patient and record all relevant information.

• Patient's Age: The patient's age is important for evaluating the history. Different age groups may have different causes for symptoms. For example, in childbearing years, bleeding is likely related to reproductive disorders. In adolescent females, bleeding with a normal ultrasound appearance may have an endocrine basis. In postmenopausal females with bleeding, an ultrasound examination often reveals a pelvic mass, with carcinoma of the genital tract being a significant consideration.

• Gravidity: Gravidity refers to the number of pregnancies a female has had. A primigravida is a woman who is pregnant for the first time, while a multigravida has had multiple previous pregnancies.

• Parity: Parity is used to describe the number of times a female has given birth to an infant weighing 500 grams (g) or more, whether alive or dead. Ultrasonography can help estimate gestational age when the weight is not known. A primipara is a woman who has given birth for the first time to an infant weighing 500 g or more, alive or dead. A multipara is a woman who has given birth two or more times to infants weighing 500 g or more, alive or dead. The term grand multipara is used for a woman who has given birth seven or more times to infants weighing 500 g or more.

• Recording Obstetric Data: Obstetric data is typically recorded using a four-digit scheme. The first number represents the number of pregnancies, the second number represents the number of premature deliveries, the third number represents the number of abortions, and the fourth number represents the number of living children. For example, 4-2-1-1 indicates four pregnancies, two premature deliveries, one abortion, and one living child.

• Parturient: A parturient is a female who is in the process of giving birth.

• Puerpera: A puerperal female is a woman who has given birth within the past 42 days.

• Menstruation: The date of the last normal menstrual period (LNMP) is important to investigate. In primary dysmenorrhea, there is typically no ultrasound-detectable pelvic mass at the onset. In secondary dysmenorrhea, there is usually a demonstrable pelvic disease that may or may not be detected on ultrasound. Changes in menstrual patterns should be carefully evaluated and differentiated from uterine bleeding unrelated to menses.

• Menstrual Disorders:

1. Menorrhagia or hypermenorrhea: Prolonged menstrual bleeding.
2. Metrorrhagia: Irregular or acyclic bleeding.
3. Menometrorrhagia: Excessive or irregular uterine bleeding between as well as during menstruations.
4. Oligomenorrhea: Reduction in menstrual frequency, with longer intervals between cycles, usually less than three months.
5. Polymenorrhea: Abnormally frequent menstruation.

• Abnormalities of bleeding during menstruation often have an endocrine origin. Intermenstrual bleeding may have ultrasound-detectable benign or malignant causes, while bleeding after intercourse or douching may have malignant causes.

• Pain: The history of pain is important in ultrasound as certain masses can cause specific types of pain. For example, localized pain in the lower abdomen may arise from the uterus or vagina. Adnexal pain is usually felt in the lower abdomen and may radiate to the medial aspect of the thigh. Sharp pain with sudden onset may be due to torsion of a pedunculated fibroid.

• Palpation of the Abdomen: Manual abdominal examination can usually detect the uterus just above the symphysis pubis in the twelfth week of pregnancy. The abdominal enlargement is typically less pronounced in nulliparas (women who have not given birth) compared to multiparas (women who have had multiple previous pregnancies).

• Changes in Uterine Size During Pregnancy: In the early weeks of gestation, the uterus increases in size in the anteroposterior direction. Over time, it becomes more globular, and by 10 to 12 weeks, it measures an average diameter of 8 cm. In pregnancies associated with inflammatory processes or cervical carcinoma, the cervix may remain enlarged from the early phase of gestation.

SONOANATOMY

The skin, adipose tissue, muscular layers, and planes of the abdominal wall can be visualized and delineated using proper gain settings.

The omentum and bowel display irregular and disorganized groups of echoes on ultrasound.

The spine is easily identified as a posterior structure that blocks sound transmission.

The normal uterus is easily identified on ultrasound.

SONOLAPAROTOMY

• In the nonpregnant female, the symphysis pubis and small intestine act as barriers, preventing ultrasound penetration of the pelvic organs and making it difficult to delineate the anatomy of the uterus and adnexal structures.

• Prior to the examination, the bladder should be distended to enhance ultrasound penetration. This is achieved by having the patient drink a large amount of fluid and instructing her not to void before the examination. A filled bladder displaces the uterus posteriorly and the small intestine superiorly, creating a transmitting window through which the ultrasound beam can be angled more effectively, leading to better visualization of the pelvic organs.

• In some cases, it may be necessary to fill the bladder using a Foley catheter to achieve proper evaluation of the uterus and pelvic region. The bladder may need to be filled and emptied in stages for optimal diagnosis.

• The distended bladder appears echo-free and is located below the symphysis pubis. The nonpregnant uterus also appears echo-poor behind the bladder at low sensitivity. The size of the normal uterus varies based on age, menstrual function, and parity. Delineating the normal ovaries and other adnexal structures is challenging, and pathologic changes may aid in their recognition.

• The use of the anterior transrectal plane (ATC) and longitudinal xiphoid pubis (LXP) systems for marking the skin in relation to bony structures has proven to be practical, particularly for procedures like amniocentesis that may involve subsequent interventions by different physicians.

• Depending on the stage of pregnancy or the size of pelvic masses, the interval between ultrasound sections may range from approximately 1 cm to 2-4 cm. The choice of sectioning and intervals is at the discretion of the examiner.

• Permanent records can be obtained using Polaroid or 90-mm film from the oscilloscope. Gray-scale images are typically obtained from the scan converter screen, and a series of pictures is taken to complete the study, including transverse sections (arranged in ascending order) and longitudinal sections (attached in sequence).

• The oscilloscope screen is divided into centimeter equivalents, and measurements on the sonogram are referenced using centimeters. Needlepoint calipers or electronic calipers with direct digital read-out systems can be used to measure specific parameters, such as the biparietal diameter of the fetal head or the anteroposterior diameter of the fetal chest.

SONOFLUOROSCOPY OF THE PREGNANT UTERUS

• The initial examination of the uterus involves rapid scanning to determine the location of the fetus, its lie, presentation, and the position of the placenta.

• This screening can be done using a real-time scanner, bistable unit, or gray-scale machine. The presence of a variable persistence oscilloscope with a rapidly fading scan image as the transducer moves through the uterus is beneficial.

• Longitudinal scanning planes provide the maximum information, while further data are obtained in the transverse plane.

• The position of the fetus, whether in breech or vertex presentation, is identified. The cranial vault appears echogenic and is surrounded by a circle of high-amplitude echoes, while the body produces a circle of lower echo amplitude. The relationship between the fetal head, body, and thorax is noted.

• Fetal respiratory excursions can be monitored using A-mode, M-mode, or real-time scanning. The fetal aorta descending into the abdomen is observed, along with its relation to the spine.

• Various organs and structures are identified, such as the liver, spleen, stomach, gallbladder, umbilical vein, kidneys (for possible anomalies), bladder, and fetal genitalia.

• The umbilical cord is followed from the placenta to its insertion into the fetal umbilicus. The position of the placenta and the presence of placental degeneration or pathological echo patterns are examined.

• Shadowing of fetal parts by the posterior placenta is common. The relationship between the placenta and the internal cervical os is identified.

• In obese or large-for-date patients, the ultrasonographer looks for multiple gestation, polyhydramnios, and other disorders. The fetal head is best studied for twin gestation using a real-time scanner.

• Small-for-date patients are examined for possible fetal anomalies, and the relationship between the size of the fetal head and body is noted. The transmission of aortic pulsation can be detected.

• After sonofluoroscopy, each fetal structure is given special attention with appropriate measurements.

SONOPHYSIOLOGY OF PREGNANCY 

DIAGNOSIS OF EARLY PREGNANCY

• Ultrasonography is used to determine early pregnancy by identifying the gestational sac or pregnancy ring within the uterine cavity.

• The gestational sac appears as a circular pattern of dense echoes and is usually located in the upper half of the uterus.

• The bladder should be distended before the examination to optimize visualization of the gestational sac.

• The gestational sac can be recognized at around 5 weeks gestation, corresponding to 7 weeks from the last menstrual period.

• Serial examinations are performed to monitor the growth of the gestational sac and detect any abnormalities.

• Around the eighth to the ninth week, a thickening of a portion of the pregnancy ring indicates the developing placenta.

• Internal echoes within the gestational sac appear after 9 weeks due to the enlarging fetal outline, and fetal heartbeat can be detected at this stage.

• By the tenth to thirteenth week, the gestational sac is no longer identifiable by ultrasound, and scattered formless internal echoes may be observed.

• Doppler ultrasound or real-time scanning can help distinguish between normal pregnancy and other conditions during the "gray zone" of pregnancy.

• The corpus luteum of pregnancy, seen as a cystic mass in the ovary, regresses as pregnancy progresses.

• Gestational age can be determined by measuring the gestational sac and fetal head, with biparietal diameter being the most accurate measurement for age determination.

• The fetal head becomes visible at around 12 to 13 weeks gestation, and its size and shape change as pregnancy advances.

• Estimation of fetal weight can be done using biparietal diameter and fetal thoracic measurement.

• Serial measurements at 3-week intervals can track fetal growth and development.

• It is important to consider variations in fetal head sizes and percentile ranks to assess intrauterine growth.

• Serial measurements between 20 and 36 weeks are conducted for accurate assessment of fetal growth and development.

• The biparietal diameter may not be reliable for fetal well-being assessment in cases of diabetic fetuses.

MEASUREMENT OF THE BIPARIETAL DIAMETER 

• Prior to the development of the ultrasonic method, physicians relied on X-ray studies (roentgenographic studies) to measure the size of the fetus and biparietal diameter.

• X-ray studies were limited to certain conditions and optimal pelvimetry criteria for accurate measurement of the fetal head in relation to the bony pelvis.

• Ultrasound allows for more accurate measurements of the fetus, regardless of its position and lie.

• Longitudinal sections are best for determining fetal presentation, locating the fetal head and fetal thorax, and monitoring changes in fetal head position or motion.

• The biparietal diameter (BPD) is a crucial measurement dependent on precise localization of the fetal lie and the angle of the fetal head with respect to the ultrasound waves.

• A-mode and B-mode measurements are made directly from the ultrasound screen, and electronic calipers or simultaneous comparisons can be used for measurements.

• Gray-scale scanners produce a thicker skull outline that needs to be reduced to depict the calvarium as a single line.

• In vertex presentation (head on the side), a longitudinal scan helps determine the angle of flexure of the head within the maternal pelvis, which can be corrected by angling the transducer in the transverse scanning plane.

• Accurate measurement of the BPD is facilitated by locating strong and equal-amplitude echoes from the near and far sides of the skull.

• Combining A-mode and B-mode with grayscale provides maximum information.

• The widest diameter of the fetal skull perpendicular to the midline echoes is considered the maximum biparietal diameter.

• Curved midline echoes can still imply a valid reading.

• Variations in gestational age determination exist between different charts and tables for fetal age evaluation based on BPD.

• Placental appearance and specific data can be used for determinations.

• If the BPD exceeds 105 mm, the study should be repeated, and hydrocephalus should be considered.

• Accurate measurements are important, and the A-mode or simple bistable mode is still considered superior.

• Tilted fetal skulls entering the pelvic inlet can result in oblique planes for obtaining the biparietal diameter, requiring adjustments by the ultra sonographer to register it accurately.

MATERNAL PELVIS

• X-ray pelvimetry is considered the best method for examining the bony pelvis, although ultrasound pelvimetry can also be performed. Indications for X-ray pelvimetry include:

1. Diagonal conjugate less than 11.5 cm.
2. Presence of diseases that affect the bony pelvis.
3. Very prominent ischial spines with a flattened sacrum.
4. Failure of progression of labor.
5. Abnormal presentations such as breech or face presentation.

• Narrowed inter tuberous diameter accompanied by a narrow sub pelvic angle.

• When using ultrasound, the fetal head can be visualized as a strong echo on the oscilloscope. Gray-scale equipment allows visualization of not only the bony structures but also the surrounding soft tissues like fat, muscle, and hair. Decreasing the gain of the ultrasound unit can help obtain a more accurate outline of the fetal head.

• The midline structure seen on ultrasound represents the interhemispheric region, including the intracranial falx cerebri and the third ventricle. Lateral to the falx, echoes likely represent the lateral portion of the anterior horn of the lateral ventricle. C-shaped structures at the junction of the frontal and facial bones represent the bony orbits.

• The fetal spine appears as strongly echogenic lines connecting the head to the thorax and continuing with a dorsal curvature throughout the rest of the spine. Ribs can be visualized as parallel lines projecting from the spine. The aorta, which lies along the spine, can be differentiated by its sharper appearance and lack of sonic shadowing.

• The fetal heart becomes functional between 35 to 45 days of amenorrhea. A-mode or M-mode ultrasound can detect fetal heart motion as early as 7 to 10 weeks of gestation. Evaluating fetal death is aided by Doppler studies coupled with real-time scanning or M-mode, as radiologic imaging signs of fetal death occur long after fetal demise.

• Monitoring the fetal heart rate can be done using a real-time scanner, but for precise evaluation, the fetal heart signal should be registered through a chart recorder. Doppler ultrasound can be used for continuous monitoring of the fetal heart rate. The normal fetal heart rate is typically 120 to 140 beats per minute.

• A unit with a multiple transducer array allows continuous tracking of the fetal heart, even if the fetus moves. The fetal heart signals are transmitted to a counting circuit and chart recorder. Simultaneously, another channel of the unit records uterine contractions. Any irregularities in the fetal heart rate can be easily registered during labor and throughout delivery. By adjusting the unit, decreases or alterations in fetal heart motion can be detected, alerting the physician to possible fetal distress.

• In fetal thorax imaging, a cross-section of the chest reveals the rounded outline of the vertebral column, including the vertebral body and neural arch elements. The fetal heart, previously discussed, helps accurately locate the fetal thorax. Rib detail may be imaged as a series of closely parallel echogenic linear structures. In late pregnancy, the fetal thorax can produce a sonic shadow over the placenta. The motion of the thorax can be demonstrated using M-mode.

FETAL ABDOMEN

• The fetal abdomen appears as a rounded image in cross-section and can be distinguished from the fetal thorax by the absence of cardiac pulsations.

• The attachment of the umbilical cord at the fetal umbilicus can be observed as an interrupted series of linear parallel echoes or as a pulsatile structure.

• The fetal liver is echogenic, and the gallbladder and stomach can be identified in the right upper quadrant as two anechoic structures.

• The umbilical vein, shorter than the gallbladder, has an anteroposterior course from the umbilicus to the region of venous union near the dorsal spine.

• The fluid-filled fetal stomach is seen as an echo-free space of variable appearance in the left upper quadrant.

• The fetal kidneys resemble miniature versions of the adult organs and have an echo-free periphery and echogenic interior. Intrauterine hydronephrosis, indicated by cystic transformation of renal outlines, can be identified.

• The urinary bladder appears as an echo-free area in the fetal pelvis and can vary in size. Fetal urination may be observed through gray scale or real-time scanning during bladder distension.

• The fetal colon may occasionally be visualized as an echo-free area in the abdominal cavity, unrelated to other cystic organs.

• Fetal weight can be estimated by measuring the biparietal diameter of the fetal head and the anteroposterior diameter of the chest. By using these measurements and a nomogram, the estimated age or weight of the fetus can be evaluated.

• In approximately 80% of cases, the estimated weight based on these measurements is within 0.5 pounds of the actual birth weight. However, there can be variations due to factors such as diabetes or malnourishment.

• Clinical data and combined ultrasonic studies can aid in making appropriate decisions regarding elective cesarean sections.

SEX DETERMINATION

• Higher-resolution gray-scale ultrasound systems allow for detailed imaging of the fetal perineum.

• To locate the perineum, the fetal orientation is noted, and landmarks such as the fetal bladder and fetal pelvis are identified.

• Sections are then made parallel to the long axis of the fetus to visualize the fetal penis or scrotum connected to the fetal perineum.

• It is important to demonstrate this physical attachment to differentiate it from fetal limbs that may simulate the rounded appearance of the scrotum.

• The fetal penis appears as a short linear echo pattern, while the scrotum is seen as an echo-free compartment with a central median septum.

• The testes within the fetal scrotal compartments appear as symmetric low-amplitude echoes.

• Female sex can be determined by the absence of the penis and scrotum in the ultrasound images.

• The optimal time for scanning to determine fetal sex is around 30 to 32 weeks of gestation.

• Determining fetal sex can be useful for genetic counseling, particularly for parents who may be at risk of having children with sex-linked disorders like hemophilia.

FET AL EXTREMITIES 

• The upper and lower limbs of the fetus are best studied using a combination of high-resolution gray-scale scanning and real-time scanning.

• Real-time scanning is particularly useful for observing the motion of the arms and legs, which helps assess gross neurologic function.

• Sonofluoroscopy, a technique that involves real-time ultrasound imaging of the uterus, can show the coordinated motion of the limbs in relation to the fetal trunk.

• Mental integration of the limb movement allows the ultrasonographer to determine which extremity is associated with the upper trunk and which belongs to the lower portion of the fetus.

• After locating and identifying the fetal limbs accurately with real-time scanning, the gray-scale ultrasound unit can be used with a 2.25 or 3.5-MHz transducer to visualize the bony structures of the arms and legs, as well as the developing digits (fingers and toes) of the fetus.

• Visualizing the phalanges (finger and toe bones) can be challenging, but with effort, the ultrasonographer may be able to obtain a clear picture indicating the presence of an adequate number of fingers or toes.

• This detailed imaging of the limbs and digits is valuable in cases of suspected fetal genetic defects or other anomalies.

FETAL MOTION

• Early fetal motion can be detected between 8 and 10 weeks' gestation using real-time scanning.

• Fetal motion at this stage consists of changes in position within the uterine cavity and limb motion without altering the fetal position in the gestational sac.

• Fetal heart motion can usually be detected at 14 weeks' gestation using real-time scanning.

• As gestation progresses, movement patterns such as head bobbing and chest wall excursion become discernable.

• Head bobbing indicates the presence of a certain degree of neurologic function in the fetal nervous system.

• Chest wall motion suggests that the fetus may be capable of breathing when born.

• Vigorous fetal motion can sometimes make it challenging to obtain accurate fetal measurements, such as thorax size and biparietal diameter.

• Real-time scanning provides the best imaging of various fetal movements.

FETAL GROWTH 

• Ultrasonic cephalometry is a valuable tool for determining fetal growth when combined with clinical correlation.

• The size of the gestational sac can be measured at the beginning of gestation to track fetal growth.

• The fetal heart can be detected using a real-time scanner as growth progresses.

• Sequential monitoring of the biparietal diameter (BPD) can provide information about fetal growth, with an average increase of 1 to 2 mm per week.

• Sudden changes in the growth rate of the fetal head may indicate pathologic conditions such as placental insufficiency or fetal death.

• In cases of twin gestation, the BPD of both fetuses should be followed, although identifying each twin can sometimes be challenging.

• BPD measurement and follow-up are particularly helpful in abnormal pregnancies associated with anencephaly or hydrocephalus.

• Ultrasonography is excellent for evaluating fetal presentation and position, especially in cases where palpation is difficult due to factors like obesity or a thick anterior placenta.

• Ultrasonography allows detailed visualization of the muscles of the anterior abdominal wall and their fascial attachments.

• The fetal head can be easily identified as a round structure on one side of the uterus using ultrasound.

• Determining fetal orientation other than cephalic presentation is crucial for obstetric management.

• Longitudinal and horizontal ultrasonic scans help determine fetal attitude and localization of fetal parts.

• Stronger echoes are returned from the fetal skull, making its localization easier, and midline structures such as the falx cerebri or lateral ventricles provide diagnostic information.

• Evaluation of the fetal trunk requires identification of the fetal spine and heart using a real-time scanner, followed by different sections at various levels for further evaluation.

• Bistable or gray-scale units can display the fetal heart and kidneys, and slicing at various levels helps ascertain the position of the bladder in relation to the rest of the trunk.

• Cardiac contractility can be evaluated, and the pulsations of the fetal heart (120 to 140 beats per minute) can be captured using M-mode.

• Fetal anatomy can be identified in detail using ultrasound imaging.

• In the cephalic presentation, the fetal head is localized in the lower part of the uterus, and its detection and display echo are not usually difficult.

• When the head is well flexed, the displayed echo appears perfectly circular in shape due to the section going through both the parietal and suboccipitobregmatic diameters.

• With further extension of the head, other diameters can be identified, and the head loses its circular shape.

• In breech presentation, the head is localized in the upper part of the uterus, and detecting the head and its display echoes may be challenging, particularly with excessive fetal movement.

• The fetal spine and fetal heart are the best landmarks for differentiating the fetal head from the fetal trunk in breech presentation, and longitudinal sections are valuable for detecting the breech.

• In transverse lie, the head and trunk are usually located at the same level, while fetal parts such as hands and feet are positioned above or below the trunk.

• Distinguishing transverse lie from the appearance of twins can be challenging, and obtaining multiple sections at different times is necessary for absolute certainty.

• The cause of malpresentation, such as transverse lie, should be further investigated.

• The availability of a real-time scanner simplifies the diagnostic workup for malpresentation.

DIFFERENTIAL DIAGNOSIS

• Any pelvic mass of sufficient size or enlarged uterus should be investigated for possible pregnancy.

• Simple, uncomplicated fibroids appear as echo-free spaces, but in pregnancy, fetal parts have their own echo patterns. Myomatous degeneration of fibroids usually results in internal echoes within the fibroid.

• Fibroids are firmer than the pregnant uterus and may produce an indentation on the posterior surface of the bladder. Fibroid uterus can coexist with pregnancy.

• Bladder diverticulum can be easily distinguished from a pregnant uterus.

• Recognition of a nonpregnant uterus in an abnormal position relative to other organs automatically excludes pregnancy.

• Ovarian cysts are usually echo-free, unless they are loculated.

• Carcinoma of the ovary does not have a specific echographic pattern, only the mass can be outlined.

• Ectopic pregnancy may be suggested by confirmation of the pregnancy ring outside the uterus, fetal parts seen outside the uterus, or an enlarged uterus with diffuse amorphous internal echoes and no fetal parts.

• Hydatidiform mole (molar pregnancy) typically produces symmetric uterine enlargement and has a specific echo pattern.

• Multiple pregnancy can be recognized before fetal structures are observed by the presence of multiple gestational sacs and, as the pregnancy progresses, multiple fetal heads, thoraxes, and placentas may be demonstrated.

• Spurious pregnancy or pseudocyesis, which occurs in women with an intense desire for a child, can be automatically excluded by ultrasonography, helping to convince the patient of the actual situation.

THE PLACENTA

• Ultrasonography has been a major achievement in determining the position of the placenta in the uterus with respect to the cervix.

• It allows for the study of normal and abnormal placental positions and internal structures with great accuracy.

• Precise placental localization is necessary for evaluating bleeding in pregnancy and for procedures like atraumatic amniocentesis.

• Ultrasonographic placental localization has replaced the use of radioisotopic scanning.

• The placenta is first seen around 10 weeks of gestation and may occupy between half and three-fourths of the uterine cavity.

• The placental tissue starts as moderately echogenic and later assumes a low-amplitude homogeneous echo pattern.

• Routine B-mode or grayscale imaging shows the placenta as a thick speckled band of echoes surrounded by a thin echogenic boundary representing the chorionic plate.

• Echoes inside the placenta come from the internal texture of the placenta (chorionic villi), and an echo-free zone corresponds to the amniotic fluid between the placenta and fetus.

• Transverse scans are helpful in establishing the primary site of the placenta, which is important for procedures like amniocentesis.

• Gray-scale imaging reveals new dimensions in placental growth and pathology, allowing for better visualization of internal echoes and the diagnosis of posterior placenta.

• The posterior placenta may show weak echoes due to the fetus attenuating the sound beam, resulting in sonic shadowing.

• The majority of anterior placentas tend to be on the right side of the uterus, while posterior placentas are frequently left-sided.

• The placental form can be influenced by local pressure changes, such as enlargement in the presence of edema or thinning and depression when adjacent to fetal structures or in cases of polyhydramnios.

• As gestation progresses, anechoic spaces representing blood-filled spaces appear within the placenta. Calcification of intercotyledonary septa and irregular placental calcifications may also be observed

TYPES OF PLACENTA 

• The placenta can have different positions in the uterus: anterior (along the anterior wall), fundal (in the uterine fundus), corporeal (adjacent to the body of the uterus), or posterior (against the posterior uterine wall).

• Anterior fundocorporeal placenta refers to a placenta that extends the entire length of the uterus in an anterior position.

• Posterior placentas are often shielded from the ultrasound beam by fetal parts, making visualization challenging.

• Placenta previa is diagnosed when placental tissue covers the internal cervical os. A full bladder is important for better imaging, especially in posterior placenta previa.

• To differentiate placenta previa from other conditions, such as a floating fetal head, manual compression of the head can be performed to observe changes in distance between the fetal skull and the maternal sacrum.

• Placenta previa can be marginal (lower border of the placenta reaches the internal cervical os), partial (covers a portion of the cervical os), or total (completely covers the cervical os).

• Detection of placenta previa in breech presentation or transverse lie is difficult, and proper identification of the fornix and midline is essential.

• In cases where the fetus is in vertex presentation and the placenta is posterior, the fetal head may produce sonic shadowing of the placenta, making visualization challenging. Sonofluoroscopy or repositioning of the fetal head can help improve visualization.

• Routine ultrasound in early pregnancy is recommended to locate the position of the pregnancy ring and prepare for possible placental abnormalities.

• Placental migration can occur throughout pregnancy, and the distance between the lower segment of the placenta and the cervix may increase as pregnancy progresses. After 32 weeks' gestation, the diagnosis of placenta previa is usually firm.

• Serial ultrasound examinations are important to monitor placental position and rule out placenta previa as pregnancy advances.

ABRUPTIO PLACENTAE 

• Abruptio placentae refers to the premature separation of the placenta from the uterus, leading to hemorrhage.

• Ultrasonographically, abruptio placentae appears as an echo-free zone with a crescentic shape between the placenta and the maternal uterus, corresponding to the location of the blood. There may be a bulge of the placental contour into the amniotic cavity.

• Signs of fetal distress or demise should be assessed during the ultrasound examination.

• Ultrasonography helps in ruling out other conditions and confirming the diagnosis of abruptio placentae.

• Ultrasonographic findings that support the diagnosis include exclusion of placenta previa, the presence of numerous echoes indicating placental separation, detection or exclusion of retroperitoneal hematoma, and differentiating falsely appearing abruptio placentae caused by tangential study of a normal placenta.

• The size of the placenta can increase in conditions such as multiple gestations, diabetes, syphilis, and Rh disorders.

• Placental volume may decrease in cases of fetal demise or organizing infarct.

• The development of echo-poor spaces within the near-term placenta has been suggested as an indication of fetal maturity

FETAL EVALUATION

• The analysis of amniotic fluid components is important for various antenatal diagnostic studies.

• Chromosome analysis of amniotic fluid is used to study Down's syndrome in patients with increased maternal age or a previous child with a trisomy 21 or other trisomy disorder.

• Enzyme assays in amniotic fluid are used to evaluate fetuses with suspected inherited biochemical disorders.

• Chromosomal linkage analysis is valuable for certain autosomal dominant abnormalities.

• The level of alpha-fetoprotein in amniotic fluid can aid in the intrauterine diagnosis of spina bifida or anencephalic states.

• Amniocentesis is typically performed at around 16 weeks' gestation when there is adequate amniotic fluid volume and increased cellularity.

• Placental localization is important before amniocentesis to ensure proper needle placement away from placental tissue, especially avoiding the umbilical artery and vein.

• Amniocentesis carries more risk when twins are present, but continuous visual monitoring with real-time ultrasound can guide the needle placement.

• A specially grooved needle with a Teflon sheath is used for amniocentesis to improve visibility under ultrasound.

• The needle is inserted obliquely through the skin and into the amniotic fluid, and its position is monitored using the real-time scanner.

• If the needle is not initially detected, the transducer can be angled to pick up the needle echoes.

• In some cases, the needle may appear within the amniotic cavity without fluid returning, indicating displacement of the membranes. In such cases, a gentle push on the needle can puncture the membranes under the guidance of the real-time scanner.

• Analysis of amniotic fluid components is important for antenatal diagnostic studies.

• Chromosome analysis of amniotic fluid is used to study Down's syndrome and other trisomy disorders.

• Enzyme assays in amniotic fluid evaluate suspected inherited biochemical disorders.

• Chromosomal linkage analysis is useful for certain autosomal dominant abnormalities.

• Alpha-fetoprotein level in amniotic fluid aids in diagnosing spina bifida or anencephalic states.

• Amniocentesis is performed around 16 weeks' gestation with sufficient amniotic fluid volume and increased cellularity.

• Placental localization is crucial to avoid needle placement near the placenta, including the umbilical artery and vein.

• Amniocentesis carries higher risk with twins, but real-time ultrasound monitoring helps guide needle placement.

• A grooved needle with a Teflon sheath is used for amniocentesis to enhance ultrasound visibility.

• The needle is inserted obliquely through the skin into the amniotic fluid, and its position is monitored using real-time ultrasound.

• Adjusting the transducer angle can help detect the needle echoes if not initially visible.

• If the needle appears in the amniotic cavity without fluid return, gentle pressure can puncture the membranes under real-time ultrasound guidance.

ANOMALIES OF PREGNANCY

• Ultrasonic determination of fetal demise depends on the duration of pregnancy.

• In early gestation, fetal demise may be represented by fragmentation of the gestational sac, appearing as an echo-free zone with scattered strong echoes.

• Doppler ultrasound and M-mode recording can be used to evaluate fetal viability later in pregnancy.

• Between the tenth and thirteenth week of gestation, when there is no clearly defined gestational sac or fetal head, Doppler techniques, M-mode, or the real-time scanner are used to check for fetal motion or heartbeat.

• Absence of fetal motion or heartbeat indicates fetal death.

• Echographic changes over time include a double ringlike contour of the fetal skull, irregular chest wall appearance, disorganized echo pattern within the fetal head and thoracic cavity, and grossly distorted outlines of the head and thorax.

• Air within the fetus due to internal decomposition may produce a sonic shadow.

• Certain radiologic signs of fetal death may include Spalding's sign (overlapping of skull bones due to brain liquefaction), exaggerated curvature of the fetal spine, and presence of gas in the fetus.

ASSOCIATED ABNORMALITIES IN PREGNANCY

• Polyhydramnios: It occurs when the fluid volume in the amniotic cavity exceeds 2000 ml. Clinically detectable when roughly 3000 ml of fluid is present. Ultrasonographically, a large area of sonolucency inside the uterus is noted, with separation of limbs from their usual position. The fetus appears to be floating due to the large fluid volume, and the fetal outline is sharply delineated.

• Oligohydramnios: It occurs when the fluid volume is below the normal range. Ultrasonographically, there is hyperflexion of the fetal head onto the trunk, and the expected echo-free zone of amniotic fluid surrounding the fetus is decreased. This leads to poor acoustic visualization of the external fetal contours and impaired imaging of the posterior uterine wall and retrouterine structures.

• Both polyhydramnios and oligohydramnios can reflect fetal abnormalities. Polyhydramnios is often associated with anencephaly, while oligohydramnios can occur with renal agenesis.

• Hydatidiform Mole: Ultrasonography is used to diagnose this condition, where a rapidly growing mass of grapelike structures develops instead of a normal fetus. The uterus is larger than expected for the calculated gestation. Ultrasonographically, there are no echoes from a fetus or placenta. A snowstorm echo pattern caused by numerous tissue interfaces within the uterine cavity filled with the molar vesicular mass is typically observed. Theca lutein cysts associated with hydatidiform mole can also be detected.

• Early diagnosis of hydatidiform mole is important due to its potential for malignant transformation. Gray scale or real-time scanning can be used to demonstrate fetal motion and determine the presence of a coexisting normal pregnancy.

• Other conditions that may mimic a hydatidiform mole include missed abortion with retained products of conception, multiple pregnancies before the appearance of fetal heads, ovarian tumors, and degenerating fibroids. Clinical data, laboratory tests, and ultrasonographic findings should be combined for accurate diagnosis.

• Hydropic degeneration of pregnancy is a proliferative form of fetal demise where fetal structures have disappeared and the placenta has undergone hydropic degeneration. It may mimic a hydatidiform mole, but the uterus is small for date. Chorionic gonadotrophin levels are usually not as high as in a hydatidiform mole.

• Evacuation of the uterus is necessary for confirmed cases of hydatidiform mole or other abnormal conditions associated with pregnancy.

MASSES IN PREGNANCY 

• Masses such as cervical fibroids can be easily detected using ultrasound.

• These masses have the potential to block and interfere with normal delivery.

• Ultrasound is vital for not only detecting these masses but also determining their relationship to the pregnant uterus and growing fetus.

• Ultrasound can be used to detect and follow up on cysts that may develop in the ovaries following hormone therapy.

• When pregnancy is present, ultrasound is the preferred imaging modality for assessing masses and their impact on the pregnancy.