Antenatal Investigations at 16 Weeks Gestation
Recommended Investigations at Second Trimester (16–18 weeks)
Key Point
The second trimester (16–18 weeks) is the optimal window for maternal serum screening and detailed morphology ultrasound. The 75 g OGTT for gestational diabetes screening is performed at 24–28 weeks, NOT at 16 weeks.
Investigation Schedule in Pregnancy
| Investigation | Timing | Purpose |
|---|
| Quad screen (AFP, hCG, uE3, inhibin A) | 15–20 weeks | Down syndrome, Edwards syndrome, neural tube defects |
| Detailed anomaly scan | 18–20 weeks | Structural abnormalities, fetal anatomy |
| Oral glucose tolerance test | 24–28 weeks | Gestational diabetes screening |
| Blood group & antibody | 1st visit & repeat at 28 weeks | Alloimmunization risk |
High-YieldNEET PG
The quad screen is a second-trimester biochemical marker test with detection rates of 80% for Down syndrome and 60% for Edwards syndrome when combined with nuchal translucency (NT) from first-trimester ultrasound.
Why 75 g OGTT Is NOT Done at 16 Weeks
- 1.
Physiological timing: Insulin resistance peaks at 24–28 weeks due to placental hormones (human placental lactogen, cortisol).
- 2.
Clinical utility: Earlier screening does not improve outcomes; gestational diabetes typically manifests in the second half of pregnancy.
- 3.
Guidelines: ICOG (Indian College of Obstetricians and Gynaecologists) and WHO recommend screening at 24–28 weeks.
Clinical Pearl
A universal screening approach (all pregnant women) with 75 g OGTT at 24–28 weeks is now standard in India, replacing the older risk-factor-based approach.
Repeat Blood Group & Antibody Screening
This IS recommended at 28 weeks (second visit) to detect new alloimmunization, especially in Rh-negative women. At 16 weeks, it is not routinely repeated unless the first-trimester result was abnormal.
