The effect of gestational age on the detection rate of Down's syndrome by maternal serum alpha-fetoprotein screening.

Low levels of maternal serum alpha-fetoprotein are currently being used to screen for Down's syndrome in midpregnancy. Because of the possibility that gestational age may affect the detection rate of Down's syndrome, we analyzed maternal serum AFP levels and gestational age in 51 Down's syndrome pregnancies that had been confirmed by amniocentesis or at birth, and we compared these pregnancies with 3239 screened singleton pregnancies with known normal outcomes. The highest yield of a low risk for Down's syndrome associated with maternal serum alpha-fetoprotein occurred at 16.5 to 17.5 weeks' gestation. Our data suggest that maternal serum alpha-fetoprotein screening for Down's syndrome should be done between 16 and 18 weeks' gestation, which is the gestational age currently recommended for neural tube defect screening.