A 33-year-old woman at 16 weeks gestation has an amniotic fluid AFP of 5.0 MoM with a positive acetylcholinesterase on amniocentesis. What fetal condition does this pattern most suggest?
- A Trisomy 21 (Down syndrome)
- B Open neural tube defect (myelomeningocele) or open abdominal wall defect (gastroschisis/omphalocele) ✓
- C Turner syndrome (45,X0)
- D Congenital nephrotic syndrome
Explanation
Elevated amniotic fluid AFP (>2.5 MoM) combined with positive acetylcholinesterase (AChE) is a highly specific marker for open fetal defects that expose neural tissue or bowel to amniotic fluid — principally open neural tube defects (anencephaly, myelomeningocele) and open abdominal wall defects (gastroschisis). AChE is normally confined to neural tissue; its presence in amniotic fluid indicates a breach in the fetal surface. Omphalocele (covered defect) gives elevated AFP but negative AChE. Congenital nephrotic syndrome elevates AFP without AChE. Trisomy 21 gives low AFP on serum screening.
Reference: Williams Obstetrics, 26th ed.
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Written and medically reviewed by the StethoPrep medical team.