Amniocentesis and Invasive Prenatal Testing

Prepared and reviewed by the PrepElephant Academic Review Team

First published 2 Oct 2026 · Spot an error?

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Direct answer
What you must remember
One procedure from counselling to report
The Indian regulatory overlay
Frequently asked questions
Related topics

Direct answer

Amniocentesis — transabdominal ultrasound-guided aspiration of amniotic fluid at 15-16 weeks (15-20 acceptable) with a 20-22 gauge needle and about 1 mL per gestational week — provides foetal cells for karyotype or chromosomal microarray in women of advanced maternal age (35 and over), abnormal screening or NIPT, structural anomalies, or parental translocations, with a procedure-related loss rate of about 0.5 per cent or less. Chorionic villus sampling (10-13 weeks) buys earliness at about 1-2 per cent loss and two unique risks: limb-reduction defects under 10 weeks and placental mosaicism. Third-trimester amniocentesis assesses lung maturity (lecithin-sphingomyelin ratio 2 or more), and cordocentesis from 18 weeks enables rapid karyotype and intrauterine transfusion.

What you must remember

Amniocentesis numbers: 15-16 weeks (15-20 acceptable); 20-22G needle under continuous ultrasound; discard the first 1-2 mL (maternal contamination); loss about 0.5 per cent or less in experienced hands; anti-D 300 micrograms to Rh-negative women.
Indications: age 35 or older at delivery, high-risk screen or NIPT, previous aneuploidy, structural anomaly, parental translocation, X-linked conditions.
CVS specifics: 10-13 weeks, transcervical or transabdominal; loss 1-2 per cent; before 10 weeks, transverse limb-reduction defects; 1-2 per cent placental mosaicism may need amniocentesis confirmation.
Rapid tests versus culture: QF-PCR or FISH for 13, 18, 21, X and Y reports in 24-48 hours; culture karyotype in 10-14 days; microarray adds copy-number changes.
Third-trimester amniocentesis: lecithin-sphingomyelin ratio 2 or more indicates lung maturity (about 3.5 in diabetic pregnancy), with phosphatidylglycerol adding assurance; the Indian bedside classic is the foam (shake) test.
Cordocentesis/PUBS: foetal blood sampling from the umbilical vein at 18 weeks or later — rapid karyotype, foetal anaemia assessment (middle cerebral artery Doppler first), and intrauterine transfusion in rh isoimmunisation.
Legal frame in India: the PCPNDT Act (1994, amended 2003) regulates all prenatal diagnostic techniques — registration of facilities and machines, written consent, and an absolute prohibition on disclosing foetal sex, punishable by imprisonment and fine.

One procedure from counselling to report

A 37-year-old at 16 weeks has a quad screen risk for trisomy 21 of 1 in 80. Counselling separates screening from diagnosis: NIPT refines risk, but only an invasive test gives a karyotype, and she opts for amniocentesis. Consent covers the 0.5 per cent loss risk, bleeding, leak and infection; Rh-negative women receive anti-D after the procedure. Under continuous ultrasound a pocket away from placenta and foetal face is chosen, the skin infiltrated, the needle advanced in one thrust; the first 2 mL are discarded, then 18 mL withdrawn into a dry syringe. The woman rests briefly and reports bleeding, leaking, fever or pain over 48 hours. The sample splits: QF-PCR reports a normal 46,XX or the anomaly within two days, culture confirms in two weeks, microarray when the indication was a structural anomaly. At 11 weeks the offer would have been CVS — slightly higher loss rate, nothing before 10 weeks (limb defects), mosaic results possibly forcing confirmatory amniocentesis. At 34 weeks with poorly controlled diabetes, the same needle would fetch fluid for an L/S ratio — 2.2 with phosphatidylglycerol: mature lungs.

The Indian regulatory overlay

Invasive prenatal testing in India operates under the PCPNDT Act (Pre-Conception and Pre-Natal Diagnostic Techniques Act), which every examinee must state: genetic counselling centres, laboratories and ultrasound machines must be registered; the pregnant woman's written consent is mandatory on a prescribed form; and communicating the foetal sex by any means is a punishable offence, with records auditable by district authorities. The Act's rationale is the distorted child sex ratio; in practice it means forms, consent and indication documentation for every procedure, and an absolute taboo on any remark about sex — even for X-linked conditions, where the family is counselled about the disease, not the sex. Access is the second reality: both tests concentrate in metropolitan centres, private costs are substantial, and government colleges provide the subsidised pathway; QF-PCR has shortened the wait culture once imposed. The viva trio: amniocentesis week (15-16), CVS timing risk (limb defects under 10 weeks), and the Act's prohibition.

Frequently asked questions

At what gestation and with what volume is diagnostic amniocentesis performed?

At 15-16 weeks (15-20 acceptable), aspirating about 1 mL per gestational week — typically 15-20 mL — through a 20-22G needle under ultrasound guidance.

What advantages and risks distinguish CVS from amniocentesis?

CVS at 10-13 weeks gives results a month earlier but carries a slightly higher loss rate (1-2 per cent), risk of limb-reduction defects before 10 weeks, and 1-2 per cent placental mosaicism requiring confirmation.

What does an L/S ratio of 2 signify?

Lecithin exceeds sphingomyelin two-fold — lung maturity, the threshold for safe delivery (about 3.5 prudent in diabetic pregnancy); phosphatidylglycerol adds certainty.

When is cordocentesis chosen over amniocentesis?

For rapid karyotype in urgent situations, foetal haemoglobin assessment, and therapy — intrauterine transfusion in rh isoimmunisation — from 18 weeks.

What does the PCPNDT Act require of a doctor performing amniocentesis?

Registration of facility and equipment, documented indication with written consent on the prescribed form, meticulous records, and absolute prohibition of disclosing foetal sex.