Second trimester prenatal screening may include several blood tests, called multiple markers. These markers provide information about a woman's risk of having a baby with certain genetic conditions or birth defects. Screening is usually performed by taking a sample of the mother's blood between the 15th and 20th weeks of pregnancy (16th to 18th is ideal). The multiple markers include:
Abnormal test results of AFP and other markers may indicate the need for additional testing. Usually an ultrasound is performed to confirm the dates of the pregnancy and to look at the fetal spine and other body parts for defects. An amniocentesis may be needed for accurate diagnosis.
Multiple marker screening is not diagnostic. This means it is not 100 percent accurate, and is only a screening test to determine who in the population should be offered additional testing for their pregnancy. There can be false-positive results - indicating a problem when the fetus is actually healthy or false negative results - indicating a normal result when the fetus actually does have a health problem.
When a woman has both first and second trimester screening tests performed, the ability of the tests to detect an abnormality is greater than using just one screening independently. Nearly all cases of Down Syndrome can be detected when both first and second trimester screening are used.