Second Trimester

* Urine and blood pressure (see The First Trimester)

ADDITIONAL TESTS

* Multiple Marker Screening: There are two types of this blood test that screens for Down syndrome and neural tube defects such as spina bifida and anencephaly, in which the skull does not form properly. The first is known as the triple screen: It measures levels of the pregnancy hormones hCG, estriol, and alpha-fetoprotein (AFP). The quadruple screen checks for all of these, plus inhibin-A, which can increase the accuracy of screening. All women should be offered one of these tests.

When performed: Between 16 and 18 weeks

Results and follow-up: Results are available within one to two weeks. If your levels of AFP and estriol are low and the level of hCG is high, your baby has an increased risk of Down syndrome. The triple-screen test detects about 60 percent of those affected; the quadruple screen, 75 percent. The tests also reveal about 80 percent of neural tube defects. If your results are abnormal, you'll be offered an amniocentesis (see below). These tests have a high false-positive rate: 50 out of every 1,000 women will be told that their fetus is at risk; of these, only one or two babies will actually have Down syndrome.

* Amniocentesis: A sample of amniotic fluid is withdrawn through a needle inserted into your abdomen and tested for Down syndrome, open neural tube defects, and other genetic disorders. It's offered to women who are 35 and over, or at high risk of chromosomal or genetic disorders. Between 1 in 400 and 1 in 200 women will have a miscarriage as a result of this procedure.

When performed: At 15 to 18 weeks

Results and follow-up: The test is 99 percent accurate; results are available in two to three weeks. The FISH test can also check for abnormalities at this time.

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