Design Prenatal Genetic Screening Plan

---
name: design-prenatal-genetic-screening-plan
description: Use when planning genetic testing during pregnancy — e.g., "what genetic tests during pregnancy", "NIPT vs amniocentesis", "anatomy scan timing", "Down syndrome screening options"
source: ACOG Practice Bulletin 226 (2021); ACOG Committee Opinion 693 (2017, cfDNA); SMFM Consult Series 2015; ACMG Guidelines Prenatal Diagnosis 2016
tags: [pregnancy, genetic-screening, nipt, amniocentesis, chromosomal, anatomy-scan, health]
verified: true
---

# Design Prenatal Genetic Screening Plan

Map the sequence of genetic screening and diagnostic options across pregnancy.

## Why This Is Best Practice

**Adopted by:** ACOG, SMFM, ACMG — offer all pregnant patients the full menu of screening/diagnostic options at the first visit.
**Impact:** cfDNA/NIPT detects trisomy 21 with 99% sensitivity and 99.9% specificity (Palomaki et al., Genet Med 2011) vs. 80% for serum quad screen. Structured genetic counseling increases informed decision-making and reduces unnecessary invasive testing (Bernhardt et al., 2013).
**Why best:** Patients cannot make informed decisions without a structured menu. Timing windows are narrow — missing them forecloses options.

## Steps

1. **Counseling at first visit (weeks 8–10):** offer all options; document patient's decision.
2. **First trimester screening (weeks 10–13):**
   - **cfDNA/NIPT:** screens for trisomy 21, 18, 13, sex chromosome aneuploidies; can determine fetal sex
     - Best for: everyone; especially accurate for trisomy 21
     - Limitation: screening only, not diagnostic; high BMI may cause test failure
   - **Nuchal translucency ultrasound (NT):** NT >3.5 mm = elevated risk for chromosomal/cardiac anomalies
   - **First trimester serum screen (PAPP-A + free βhCG):** combined with NT for integrated risk score
3. **Second trimester (weeks 15–20):**
   - **Quad screen** (AFP, hCG, estriol, inhibin A): if NIPT declined or unavailable; detects ~80% of Down syndrome
   - **Anatomy ultrasound (18–20 weeks):** structural survey — brain, spine, heart, face, limbs; cervical length
4. **Diagnostic testing — offer if screening positive, advanced maternal age (≥35), or patient preference:**
   - **CVS (chorionic villus sampling):** weeks 10–13; 1 in 300–500 miscarriage risk; provides diagnosis
   - **Amniocentesis:** weeks 15–20; 1 in 300–500 miscarriage risk; diagnoses chromosomal and single-gene disorders
5. **GBS screen (weeks 35–37):** rectal-vaginal swab (see design-group-b-strep-protocol).
6. **Document**: all offered tests, patient decisions, and results in the chart.

## Rules

- NIPT is screening, not diagnostic — a positive result requires confirmation with amniocentesis before any irreversible decision.
- Advanced maternal age (≥35) increases aneuploidy risk but is not by itself an indication for invasive testing.
- Carrier screening (cystic fibrosis, spinal muscular atrophy, hemoglobinopathies) is separate from aneuploidy screening — offer preconception or at first visit.

## Examples

**Low-risk 28-year-old:** NIPT at 12 weeks → anatomy scan at 19 weeks → GBS at 36 weeks.
**High-risk (prior trisomy 21, age 38):** NIPT + NT at 12 weeks; consider amniocentesis; detailed anatomy scan at 20 weeks; echocardiogram at 22 weeks.

## Common Mistakes

- Ordering NIPT at 8 weeks — too early; fetal fraction insufficient before 10 weeks.
- Assuming NIPT covers all genetic conditions — it screens aneuploidy only; does not diagnose single-gene disorders.
- Skipping anatomy scan because NIPT was negative — NIPT does not detect structural anomalies.

> **Health Disclaimer:** This is not medical advice. Genetic screening decisions are deeply personal and involve complex medical, ethical, and personal considerations. Consult your healthcare provider and a genetic counselor before any diagnostic procedure.