data on maternal serum alpha-fetoprotein ( afp ) levels at 13-24 weeks ' gestation in 46 twin pregnancies with open neural tube defects ( 22 with anencephaly , 24 with open spina bifida ) and 169 unaffected twins were used to estimate the detection and false-positive rates associated with different cut-off levels .
C0027794
neural tube defects
C0002902
anencephaly