Very low alpha-fetoprotein in Down syndrome maternal serum screening

Objective To establish the frequency of very low maternal serum AFP and to differentiate congenital AFP deficiency from those diseases known to be associated with low AFP. Methods AFP values below 2 µg/L and borderline values up to 3 µg/L were retrospectively analysed in 839 773 singleton pregnancies included in a programme for routine screening of trisomy 21 maternal serum markers. Results Serum AFP was undetectable (≤2 µg/L) in 8 cases, giving a frequency of 1/105 000. The calculated risk of Down syndrome was ≥1/250 in 5 cases. Fetal karyotype was normal. Seven of these pregnancies went to term (39–41 weeks) uneventfully, and birth weight was normal (3050–4110 g). In the 8th case, fetal death occurred at 35 weeks due to severe maternal diabetes. AFP levels between 2.1 and 3.0 µg/L were noted in 7 other cases. The calculated risk of Down syndrome was ≥1/250 in 5 cases, and fetal karyotype was normal. Pregnancies went to term in 4 cases (33–41 weeks), and birth weight was normal (3000–3380 g). In 3 cases, low hCG (