Common Prenatal Tests 101

Maternal Blood Test

What does it test for?

Maternal blood tests show whether the baby is at higher-than-average risk for certain birth defects of the brain and spine (called neural tube defects) and Down syndrome (a chromosomal birth defect).

When is it done?

It's usually performed at about 16 weeks.

How is it performed?

A blood sample is taken and is then measured for the levels of at least three substances (a protein called alpha-fetoprotein and two hormones) in your blood.

What if the results are abnormal?

Nine of ten women receive normal results and breathe a sigh of relief. But if you're one of the 10 percent with abnormal results, don't panic -- chances are the baby is fine. Because this is a screening test, many more babies are shown to be at risk than actually have problems. In fact, for every 100 women with abnormal results, only 2 or 3 have a baby with birth defects. In most cases, abnormal test results occur because your baby is either a few weeks older or younger than originally thought, which throws off the reading. So if you have an abnormal result, the first step your healthcare provider will take is to give you an ultrasound to recalculate your baby's age. The ultrasound will also show if you're carrying twins -- another common cause of an abnormal blood test. If your baby's age or the fact that you're carrying multiples explains your result, you probably won't need any further tests.

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