Second Trimester: Maternal Serum Screen
Between 15 and 20 weeks' gestation, the mother may be given the option to have a maternal serum screening test for Down syndrome and open neural tube defects, such as spina bifida. Included in the screening panel are tests for alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), unconjugated estriol, and sometimes inhibin A. (Depending the number of markers used, the test may also be called a triple marker or a quad marker test.) This blood screening panel provides an opportunity to assess the risk of these conditions occurring without performing the more invasive amniocentesis procedure. In most cases, only those patients whose blood screening results indicate a potential problem need to be referred for amniocentesis.
During the second trimester, it is expected that the levels of AFP and unconjugated estriol will increase, the amount of hCG will decrease, and the amount of inhibin A will stay relatively constant.
- AFP is produced by the fetus and then crosses into the mother's blood. A fetus with a neural tube defect has an opening in its spine or head that allows an increased amount of AFP to pass into the mother's blood stream. An elevated AFP could also result from multiple fetuses, miscalculation of gestational age, an abdominal wall defect, or an unknown reason. An ultrasound may be requested to determine the fetus' age and confirm the number of fetuses.
- In pregnancies where the fetus is carrying the chromosomal defect that causes Down syndrome, the results of the triple or quad marker screen tend to show decreased levels of AFP and unconjugated estriol and increased levels of hCG and inhibin A.
The test results are interpreted based on the mother’s age, weight, and ethnic background to assess the risk of the baby having neural tube defects or a chromosome disorder. Of all reported cases of elevated AFP, only a very small percentage of babies truly have a defect. But if the test result is abnormal, a genetic counselor should be consulted to discuss an amniocentesis to further assess the likelihood of a birth defect.