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:
- Alpha-fetoprotein screening (AFP), also called maternal serum AFP (MSAFP), is a blood test that measures the level of alpha-fetoprotein (a protein normally produced by the fetal liver) present in the amniotic fluid (fluid surrounding the fetus). This fluid crosses the placenta into the mother's blood, where it can also be tested. Abnormal levels of AFP may signal multiple pregnancies or congenital defects of the fetus, such as spina bifida and Down syndrome.
- hCG, otherwise known as human chorionic gonadotropin hormone, is produced by the placenta.
- Estriol is a hormone produced by the placenta.
- Inhibin is a hormone produced by the placenta.
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 it is only a screening test to determine which pregnant women should be offered additional testing during their pregnancy. There can be false-positive results, which indicate a problem when the fetus is actually healthy, and false-negative results, which indicate 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 chances that the tests will accurately detect an abnormality is greater than when just one screening is used independently. Nearly all cases of Down syndrome can be detected when both first and second trimester screenings are used.