USG vs NIPT blood test vs 20-week anomaly scan
Quick summary of pregnancy scans and NIPT
- These tests are complementary, not either–or.
- NIPT is a highly accurate screening blood test for common chromosome conditions (T21/18/13 ± sex-chromosome differences).
- Can be taken any time from 10 weeks 0 days (10+0) of pregnancy (~2.5 months).
- High-accuracy screening for T21/T18/T13 (± sex-chromosome differences after counselling).
- Not diagnostic—high-chance results need CVS/Amniocentesis to confirm.
- Ultrasound (USG) imaging is recommended in different stages, throughout pregnancy.
- Ultrasound (USG) schedule:
- Early viability/dating scan: 6–10 weeks (~1.5–2.5 months)
- Nuchal translucency (NT) window: 11+0 to 13+6 weeks (~2.5–3.5 months)
- 20-week anomaly scan (18–22 weeks): a detailed ultrasound for physical/anatomy differences (brain, spine, heart, kidneys, limbs, face, abdomen). ACOG recommends this for all pregnancies, even if NIPT is low-risk.
- Growth/Wellbeing scans (if advised): typically 28–32 weeks and/or 34–36 weeks
India compliance: Disclosing foetal sex is illegal under the PC-PNDT Act. Neither ultrasound nor NIPT at RxDx Clinics NM Medical will reveal sex.
Side-by-side comparison
| Feature | NIPT (cfDNA blood test) | Routine Ultrasound (USG) | 20-week Anomaly Scan (targeted USG) |
|---|---|---|---|
| What it is | Mother’s blood test analysing placental DNA fragments | Imaging of the baby & pregnancy structures | Detailed structural scan for specified conditions |
| When | From 10 weeks onward | Throughout pregnancy (dating, NT 11–13+6, growth) | 18–22 weeks (offered to everyone) |
| Main purpose | Screening for T21, T18, T13 (± sex-chromosome conditions after counselling) | Confirm/monitor pregnancy, dates, wellbeing; NT helps refine chromosomal risk | Screening for physical/anatomy conditions (e.g., brain/spine/heart) |
| What it can find | High chance of selected chromosomal aneuploidies | Viability, number of babies, placenta/liquor, growth; NT → risk estimate | 11 specified physical conditions; may also spot signs suggestive of aneuploidy |
| Accuracy | Higher detection & lower false-positive than serum screens for T21/18/13; not diagnostic | Not a chromosome test; may see “soft markers” only | Finds many—but not all—structural conditions; not a chromosome test |
| Risk to baby | None (standard blood draw) | None (diagnostic ultrasound) | None |
| If abnormal | Genetic counselling → offer CVS/Amnio to confirm | Targeted follow-up or consider NIPT/diagnostics | Targeted fetal medicine review ± CVS/Amnio |
| Key limits | Can miss structural problems; no-call possible with low foetal fraction | Cannot rule out chromosome conditions | Cannot rule out all anomalies; some appear later |
Sources: ACOG/NIPT overview and guidance; NHS/UK programmes for timing and scope of anomaly scan; FMF/NHS for NT timing.
When should we do each?
dating/viability USG (your doctor will time this).
≥10 weeks:
NIPT if you choose it.
11–13+6 weeks (optional):
NT ultrasound (risk estimate + early anatomy review).
18–22 weeks:
20-week anomaly scan (recommended for all pregnancies even if NIPT is low-risk).
Frequently Asked Questions (FAQ)
Can NIPT replace the 20-week scan?
No. Even with a low-risk NIPT, you should still have the second-trimester anatomy scan because structural differences can occur with or without chromosome conditions.
Does the 20-week scan check chromosomes?
No. It checks anatomy. It may find clues that raise or lower suspicion, but chromosome changes need screening (e.g., NIPT) or diagnostic tests to confirm.
What if my NIPT is high-risk or the scan finds a concern?
You’ll be offered genetic counselling and a confirmatory diagnostic test (CVS/Amniocentesis). Your doctor will guide you on timing and options.
Can you tell the baby’s sex?
No. Sex disclosure is illegal in India (PC-PNDT). Our reports and scans do not reveal sex.
Takeaway
- Choose NIPT if you want an early, non-invasive screen for common trisomies.
- Always do the anomaly scan at 18–22 weeks for a thorough structural check.
- Ultrasounds before and after help with dating, growth, placenta, and overall wellbeing.
- These tests work together to give the best picture of your baby’s health.
