Birth Defects Testing

What Are the Tests?

Two types of tests—screening and diagnostic—are done during pregnancy to look for birth defects. Screening tests see whether your baby is likely to have birth defects; if a screening test is positive—meaning a problem may be present—a diagnostic test is done to confirm or rule out the problem. A negative screening test means it is unlikely that the baby has a birth defect. However, it does not guarantee a normal pregnancy or baby.

Screening tests

• Nuchal fold ultrasound and a blood test. Though not yet widely available, this combination of screening tests is done in the late first trimester to look for Down syndrome. The ultrasound uses sound waves to measure the thickness of the fetus's neck (nuchal translucency). The blood test measures the levels of free beta-hCG and a protein called pregnancy-associated plasma protein A (PAPP-A). The sensitivity of this screening test is about the same as that of the maternal serum quad screening, which is done later in pregnancy.¹
• Maternal serum triple or quadruple (quad) screening. This screening can be done in the second trimester, between 15 and 20 weeks of pregnancy. These blood tests check the amounts of three or four substances in a pregnant woman's blood. The results estimate the chance that your fetus has Down syndrome or other health problems. The triple screen checks the levels of alpha-fetoprotein (AFP), beta human chorionic gonadotropin (beta-hCG), and a type of estrogen (unconjugated estriol, or uE3). The quad screen checks these substances and the level of the hormone inhibin A. The levels of these substances—along with a woman's age and other factors—help the doctor estimate the chance that her baby may have certain problems or birth defects.
• Ultrasound. This test shows the developing baby and is often done at 18 to 20 weeks of pregnancy. Doctors can use an ultrasound to screen for some chromosome problems, such as Down syndrome. Ultrasound also can be used to help find structural problems of the heart, spine, abdomen, or other areas of the body.

Diagnostic tests

• Chorionic villus sampling (CVS). Doctors can use this test to look at cells in the placenta. CVS can be done in the first trimester of pregnancy, between 10 and 12 weeks. A doctor collects a sample of chorionic villus (placental) cells by putting a small tool into the uterus through the vagina or by putting a needle through the belly into the uterus. The test can be used to find many family diseases, such as hemophilia and sickle cell disease, and chromosomal birth defects such as Down syndrome. However, it cannot find neural tube defects.
• Amniocentesis. This test looks for many chromosomal problems by looking at cells in the amniotic fluid around the baby. A doctor puts a needle through the belly and into the uterus to collect amniotic fluid. This test is done in the second trimester, between 15 and 20 weeks (usually around week 16). Amniocentesis also can help find neural tube defects, such as spina bifida.

When CVS is done by a highly trained provider, the chance of it causing a miscarriage can be as low as 1 in 400. This is based on one study. Other studies have shown higher risks of miscarriage after CVS.²

For more information, see the medical tests Fetal Ultrasound, Alpha-Fetoprotein in Blood, Chorionic Villus Sampling, and Amniocentesis.

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Author:	Jan Nissl, RN, BS	Last Updated: May 29, 2006
Medical Review:	Renée M. Crichlow, MD - Family Medicine Siobhan M. Dolan, MD, MPH - Reproductive Genetics
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