Abnormality Screening with Obstetrical Sonography
Using a transducer, a transvaginal probe or Doppler technology, obstetrical sonographers read the presence, absence and frequency of sound waves as they reflect back images from within the pregnant woman. This increasingly accurate technology allows sonographers to provide pregnant women and their physicians with invaluable insight into the health and development of a fetus.
Now routine, obstetrical sonography reveals many details about the growing fetus, including its development, sex, age and several potential anomalies. Abnormality screening, such as that approved by The American College of Obstetricians and Gynecologists, although able to identify prenatal problems, typically provides reassurance to the pregnant patient.
First Trimester
Pelvic pain and bleeding often provide the impetus for performing a first trimester obstetrical sonogram as they may be indicators of any of a variety of abnormalities. The American Institute of Ultrasound in Medicine recommends the following occur with any first trimester sonogram: identify the presence and location of the gestational sac as well as the yolk sac and embryo (if possible), record the crown-rump length (if possible), identify the presence or absence of cardiac motion, note the number of fetuses, assess appropriate fetal anatomy, evaluate the structures of the cervix and adnexal (including any masses) and assess the nuchal region (if possible).
A variety of abnormalities may be identified during the first trimester. One common maternal abnormality is a corpus luteum (pelvic mass) that will typically present posterior through transmission, and because of variability, is best examined transvaginally.
Sonographic abnormality screening within the first three months will also frequently identify an ectopic pregnancy. Blood in the peritoneal cavity may indicate this condition, although a ruptured corpus luteum cyst may also be the culprit. The relative echogenicity of the walls of the different masses will help the sonographer distinguish between a tubal ectopic pregnancy and a corpus luteum.
If an intrauterine gestational sac is present, ectopic pregnancy may be ruled out, although caution should be exercised in making the diagnosis unless a definite embryo or yolk sac is visualized. In any event, the mere absence of a gestational sac is not enough to support an ectopic diagnosis, which requires a more specific finding such as: live extrauterine pregnancy, extrauterine gestational sac (not to be confused with a hemorrhagic cyst), extra-ovarian tubal ring or a complex adnexal mass (not attached to an ovary). Finally, color Doppler flow can help reveal the “ring-of fire” pattern characteristic of an ectopic pregnancy.
A less common abnormality detectable during the first trimester is a molar pregnancy, where a non-viable fertilized egg attaches to the uterine wall. Commonly this will be demonstrated on the sonogram with multiple cystic spaces, intrauterine anechoic fluid or an echogenic mass that resembles an edematous placenta. Another abnormality that may be visualized early in pregnancy is a sub-chorionic hemorrhage (SCH). Where the trophoblast is partially detached from the uterine wall, typically a hematoma will extend to the margin of the placenta; visually, the SCH echo texture will decrease as the fetus develops.
One chromosomal abnormality, trisomy 21 (Down’s syndrome), may be detected during the first trimester with a sonogram using nuchal translucency. Additionally, structural fetal abnormalities such as anencephaly can be visualized with sonogram during the first three months of pregnancy.
Second and Third Trimesters
Many women do not receive an ultrasound until they are well into the second trimester. The standard mid-term examination is typically thorough and includes evaluating fetal growth and number, fetal presentation and heart activity. Additionally, a number of other factors that indicate the presence of an abnormality are also observed, including significant discrepancy between uterine size and clinical dates, pelvic mass, suspected molar or ectopic pregnancy, amniotic fluid evaluation and placental abruption.
The fetal spine is typically observed with transverse and sagittal views, although the quality of the images relies heavily on the position of the fetus. Spinal malformations that may be visualized include sacral agenesis and spina bifida; one of the neural tube defects, spina bifida is highly detectable and is typically characterized by the “banana sign” of cerebellar deformity. Hydrocephalus, or water on the brain, may be diagnosed where the choroid is dangling and a separation between the choroid plexis and the ventricle is visible.
The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends extending the basic cardiac exam to include aortic and pulmonary outflow to allow greater detection of major cardiac malformations, such as transposition of the great arteries, truncus arteriosus and tetralogy of Fallot.
Down syndrome is identified in the second trimester with soft markers, structural abnormalities and adjunct characteristics. Together these include nuchal fold thickness (as opposed to translucency), hypoplastic nasal bone, echogenic bowel, shortened humerus, cardiac defects (ASD, VSD and AVSD), abdominal differences and brachydactyly.
Gastroschisis (an abdominal herniation of the fetal bowel that sometimes includes portions of the liver and stomach) will result in an abdomen of smaller circumference than would otherwise be expected, and is often best seen by using color Doppler to place the umbilical cord in relation to the hernia. Other abdominal abnormalities (such as enteric cysts) may be demonstrated by atypical fluid collections of the bowel.
For both diagnostic purposes and to alleviate patient anxiety, obstetrical sonography provides a safe, often non-invasive method to identify anomalies. Whether conducted in the first trimester, second or even third, abnormality screening with obstetrical sonography is a critical tool for providing the best prenatal care.
Further Reading
The American College of Obstetricians and Gynecologists
The American College of Radiology
The Association for Medical Ultrasound
The International Society of Ultrasound in Obstetrics and Gynecology
