Ultrasound (sonogram) is used to visualize the developing fetus, placenta, and uterus during pregnancy.  Ultrasound is sent into the body from a scanning instrument (transducer) placed on your skin. The sound is reflected off structures inside your body and is analyzed by a computer to make an image of these structures on a monitor, which is similar to a television screen. Ultrasound is painless and there are no known harmful effects associated with the medical use of sonography.

Ultrasound has different uses at different times during pregnancy.  During the first trimester, ultrasound is used to predict an accurate due date, establish twin and triplet pregnancies. An ultrasound performed between 11 weeks 3 days and 13 weeks 6 days can be used as a screening tool for chromosomes conditions such as Down syndrome by measuring the nuchal translucency thickness and evaluating the nasal bone.

A thorough ultrasound at 18-20 weeks of pregnancy is used to rule out fetal anomalies or birth defects.  The sonographer (ultrasound technician) looks carefully at all of the developing structures of the fetus.  A specific type of ultrasound, called a fetal echocardiogram, may be ordered to attain a detailed look at the heart when there is concern about a heart defect. 
A detailed ultrasound can be used to screen for chromosome abnormalities.  Birth defects such as cardiac malformations are associated with an increased risk for chromosomal abnormalities.  In addition there are features, called "markers", that are associated with chromosome abnormalities.  For example, markers that are associated with Down syndrome include nuchal fold (thickness at the back of the neck), short limbs, echogenic bowel (brightness of the bowel), echogenic foci (bright spots in the heart), and pyelectasis (extra fluid in the kidneys).  Ultrasound performed at 18-20 weeks identifies birth defects and/or ultrasound markers in as many as 50-60% of babies with Down syndrome.  Ultrasound will NOT identify markers for Down syndrome in all affected fetuses.  When ultrasound views of the baby are technically limited, the detection of Down syndrome by ultrasound is reduced.

An ultrasound may be performed in the third trimester to monitor the baby's growth, follow a suspected or known defect, check for complications in high-risk women (for example, those with diabetes or multiple gestations), and prepare for delivery. Doppler ultrasound may be used to study the blood flow in fetal blood vessels.

It is important to know that ultrasound is a screening tool.  It is not able to detect all birth defects, chromosome abnormalities, or other causes of intellectual disability.  As in all screening tests, there are false-positives and false-negatives with ultrasound.  Identification of a marker by ultrasound does not mean that the baby definitely has an abnormality.  Many healthy babies are found to have markers on ultrasound, representing normal variation.  At the same time, a normal ultrasound does not rule out all abnormalities. Genetic counseling to review the implications of various ultrasound findings is available.

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