Prenatal Screening Tests
Overview
Prenatal screening tests estimate the probability of chromosomal abnormalities and neural tube defects — they do NOT confirm them. First-trimester screening (weeks 10-13) pairs nuchal translucency ultrasound with maternal serum free beta-hCG and PAPP-A. The quad screen (weeks 15-20) measures AFP, hCG, estriol, and inhibin A. Cell-free fetal DNA can be drawn as early as 10 weeks for trisomies 13, 18, and 21 — but remains a screen, not a diagnosis. An abnormal screen prompts counseling and referral for diagnostic testing (amniocentesis or CVS), never a diagnosis on its own.
Indications
Interpretation
Direction of AFP is the pivot point. Elevated AFP points to an open structural defect; low AFP (with high hCG and inhibin A) points to Down syndrome. Every result is a probability, not a diagnosis.
First-trimester screen vs quad screen
First-trimester screen
- Timing
- Weeks 10-13
- Serum markers
- Free beta-hCG, PAPP-A
- Ultrasound
- Nuchal translucency
- Detects
- Trisomy 21 risk
Quad screen
- Timing
- Weeks 15-20
- Serum markers
- AFP, hCG, estriol, inhibin A
- Ultrasound
- None
- Detects
- Neural tube defects + trisomy 21 risk
During — Monitoring
Patient Teaching
Clinical Pearl
AFP goes UP for open defects (open tube leaks protein out) and DOWN for Down syndrome — the direction of AFP tells you the worry. And a screen flags risk; it never makes the diagnosis.