What are prenatal tests?

Prenatal tests can determine a variety of things about the overall health of a mother and her unborn baby. Certain tests are routine and almost all pregnant women get these tests as part of their prenatal care. Routine prenatal tests, such as blood and urine tests, can determine a mother's blood type, whether she suffers from gestational diabetes, or is anemic, and whether she is immune to certain diseases or has a sexually transmitted disease.

Other tests can tell a lot about an unborn baby's health, specifically whether the baby has a genetic defect or other birth defects. Some common tests used to help screen for chromosomal abnormalities or other birth defects are ultrasound, alpha-fetoprotein testing, amniocentesis and chorionic villi sampling.

In the first-trimester, using both nuchal translucency (NT), an ultrasound exam that measures the thickness at the back of the neck of the fetus, and a blood test is an effective screening method. Neural tube defect screening should be offered in the mid-trimester to women who elect only first-trimester screening for Down syndrome. When getting a NT test, look for medical centers with staff who have received special training in NT measurement for Down syndrome and have state-of-the-art ultrasound equipment.

Why would I need a prenatal test? What is a high-risk pregnancy?

While some prenatal tests are routinely recommended, others might be offered to a woman whose pregnancy is considered high risk. For example, women found to be at increased risk of having a baby with Down syndrome with first-trimester screening should be offered genetic counseling and the option of CVS or mid-trimester amniocentesis.

A high-risk pregnancy is when there is a potential for illness or complication for the mother or baby. Common factors in high-risk pregnancy are:

• Either parent has a family history of malformation, mental retardation, chromosome abnormality or neural tube defect.
• There is a maternal history of diabetes, hypertension or other chronic disease.
• There have been previous complications with pregnancy or a history of miscarriage or stillbirth.
• The current pregnancy is a multiple birth (twins, triplets, etc.).
• The mother was exposed to a harmful chemical agent during pregnancy (for example, radiation).
• Maternal screening tests produced abnormal results.

However, your doctor may offer these tests to you even if you have no apparent risk factors for problems in pregnancy. For instance, the American College of Obstetricians and Gynecologists now recommends that all pregnant women be screened for Down's syndrome - regardless of their age - before the 20th week of pregnancy.

Talk to your doctor or a genetic counselor about what your prenatal tests measure, how reliable they are, the potential risks and your options should the results indicate a defect or disorder.