Why is screening for neural tube defects not carried out at 14 weeks?

Although second trimester Down's syndrome screening can start at 14 weeks gestation, neural tube defect screening only starts at 15 weeks. AFP does not discriminate well between open spina bifida pregnancies and unaffected pregnancies before 15 weeks (1). The parameters used in Alpha are published from 15 to 22 weeks gestation (2)


1. Report of UK Collaborative Study on Alpha-fetoprotein in relation to neural tube defects (1977)

2. Maternal serum screening for Down's syndrome: the effect of routine ultrasound scan determination of gestational age and adjustment for maternal weight (1992)