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A User's Guide to Prenatal Tests

A User's Guide to Prenatal Tests

First Trimester Tests continued...

Urine tests: You'll also be asked early on for a urine sample so that your doctor or midwife can look for signs of kidney infection and, if necessary, to confirm your pregnancy by measuring the level of hCG, a hormone secreted by the placenta. (A blood test to confirm pregnancy may be used instead.) Urine samples will then be collected regularly to detect glucose (a sign of diabetes) and albumin (a protein that indicates pre-eclampsia, which is pregnancy-induced high blood pressure).

Cervical swabs: You'll also have a Pap smear to check for cervical cancer, and vaginal swabs may be taken to test for chlamydia, gonorrhea and bacterial vaginosis, an infection that can cause preterm birth. Treatment of these conditions will prevent complications for your newborn.

Chorionic villi sampling (CVS): If you're 35 or older or have a family history of certain diseases, you'll be offered this optional, invasive test between 10 and 12 weeks, which can detect many genetic defects, such as Down syndrome, sickle-cell, cystic fibrosis, hemophilia, Huntington's chorea and muscular dystrophy. The procedure involves either threading a tiny catheter through your cervix or inserting a needle into your abdomen to obtain a tissue sample from the placenta. The procedure carries a 1% risk of inducing miscarriage and is about 99% accurate in ruling out certain chromosomal birth defects. But in contrast to amniocentesis, it does not help in detecting open spine disorders, such as spina bifida and anencephaly, or abdominal wall defects.

A promising new procedure that combines a blood test with an ultrasound to screen for Down syndrome may be available between 10 and 14 weeks. The results of a blood test that measures either hCG and PAP-A (pregnancy-associated protein A) in maternal blood are used with an ultrasound measurement of the fetus' skin at the back of its neck (called nuchal-translucency). The procedure may be able to pick up a substantial portion of Down syndrome cases and other genetic diseases, says Dr. John Larsen, professor of obstetrics, gynecology and genetics at George Washington University. However, as with all screening methods, a more invasive diagnostic technique like CVS is used to follow up a positive screen.

Second Trimester Tests

Here are the prenatal tests you may face in the second trimester:

Maternal serum alpha-fetoprotein (MSAFP) and multiple marker screening: One or the other is offered routinely at 15 to 18 weeks. The MSAFP test measures the level of alpha-fetoprotein, a protein produced by the fetus. Abnormal levels indicate the possibility (not existence) of Down syndrome or a neural-tube defect such as spina bifida, which can then be confirmed by ultrasound or amniocentesis.

When blood drawn for the MSAFP test is also used to check levels of the hormones estriol and hCG, it's called a multiple-marker screening. The multiple marker test significantly boosts the detection rates for Down syndrome. The test picks up about 80% of neural tube defects and 70% of Down syndrome, but many women will have a false-positive screening. About 3% to 5% of women who have the screening test will have an abnormal reading, but only about 10% of those women will have a child with a genetic problem.

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