Prenatal screening, tests

This is the screening ultrasound usually offered to low-risk patients. A routine ultrasound can determine your baby’s due date, show if you are carrying twins and show your baby's body. It can be used to detect one-third to one-half of birth defects. Because each baby is different, ultrasound findings vary from one baby to another. If a routine ultrasound is abnormal, a comprehensive ultrasound is offered.

Comprehensive ultrasound identifies many major birth defects. About 95 percent of babies with open neural-tube defects are detected this way. Ultrasounds also can identify genetic problems. About 40 percent to 50 percent of babies with Down syndrome have markers that can be seen on an ultrasound. Comprehensive ultrasound is done between 18 weeks and 20 weeks of pregnancy, if needed.

A maternal serum quad screen is done between 16 weeks and 18 weeks of pregnancy. It helps identify pregnancies that may be at increased risk for neural-tube defects or some genetic problems. It does not diagnose or rule out these problems. The quad screen measures four substances normally found in the blood of pregnant women.

Amniocentesis is more than 99 percent accurate in diagnosing or ruling out Down syndrome and other genetic problems. It also can help diagnose open neural-tube defects. Amniocentesis is done after 13 weeks of gestation. It is offered to women who will be age 35 or older when the baby is due, because there is an increased risk of genetic problems as women age.

 Amniocentesis (PDF)

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Chorionic villus sampling (CVS) tests for genetic problems. It cannot detect open neural-tube defects. CVS occurs between 10 weeks and 12 weeks of pregnancy, and has about a 1 percent risk of miscarriage. It is not offered to all women because of this risk.

Gestational diabetes occurs only during pregnancy. Between 3 percent to 5 percent of pregnant women develop this condition. It is caused by increased hormones in the bloodstream which, in turn, cause blood glucose (sugar) levels to rise. Screening occurs between 24 weeks and 28 weeks of pregnancy.

 Gestational diabetes (PDF)

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