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Health & Pregnancy

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A User's Guide to Prenatal Tests

A User's Guide to Prenatal Tests

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.

Ultrasounds: A sonogram may be offered between 18 and 20 weeks for a variety of reasons, including verifying a due date, checking for multiple fetuses, investigating complications such as placenta previa (a low-lying placenta) or slow fetal growth, or detecting malformations like cleft palate. During the procedure, a device is moved across the abdomen that transmits sound waves to create an image of the uterus and fetus on a computer monitor. New three-dimensional sonograms provide an even clearer picture of your baby, but they aren't available everywhere and it's not clear whether they're any better than two-dimensional pictures in contributing to a healthy pregnancy or birth.

Glucose screening: Typically done at about 25 to 28 weeks, this is a routine test for pregnancy-induced diabetes, which can result in overly large babies, difficult deliveries and health problems for you and your baby. This test measures your blood-sugar level an hour after you've had a glass of soda. If the reading is high, which happens about 20% of the time, you'll take a more sensitive glucose-tolerance test, in which you drink a glucose solution on an empty stomach and have your blood drawn once every hour for three hours.

Amniocentesis: This optional diagnostic test is usually performed between 15 and 18 weeks (but can be done earlier) for women who are 35 or older, have a higher-than-usual risk of genetic disorders, or whose AFP or multiple-marker screen test results were suspicious. The procedure is done by inserting a needle through the abdomen into the amniotic sac and withdrawing fluid that contains fetal cells. Analysis can detect neural tube defects and genetic disorders. The miscarriage rate varies depending on the experience of the physician, averaging about 0.2% to 0.5% at 15 weeks and 2.2% at 11 to 14 weeks, but the test can detect 99% of neural-tube defects and almost 100% of certain genetic abnormalities.

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