The legal position in India, first
Under the Pre-Conception and Pre-Natal Diagnostic Techniques Act, communicating the sex of a fetus is a criminal offence in India. Registered laboratories suppress that information from NIPT reports, and asking for it puts both you and the clinician at legal risk. Any provider offering to reveal fetal sex is operating illegally. This is not a limitation of the technology but a deliberate legal safeguard against sex-selective abortion.
Why twins are harder to analyse
NIPT works by measuring the proportion of chromosomal material in circulating DNA. In a singleton pregnancy, one placenta contributes a measurable fetal fraction. In twins, two placentas each contribute, and they rarely contribute equally. Several consequences follow:
- Lower fetal fraction per twin. The total fetal DNA is higher, but each twin's individual share is smaller, which makes a chromosomal excess harder to detect.
- Higher test failure rate. Inconclusive results are more common in twins and often require a repeat sample.
- No way to identify which twin. A high-risk result tells you the pregnancy is at risk, not which baby, which complicates the decision about invasive testing.
- Vanishing twin effect. If one twin was lost early, its DNA can persist for weeks and produce a misleading result. Always disclose a known vanishing twin.
Identical versus non-identical matters
Monozygotic (identical) twins share a genotype, so NIPT behaves much as it does in a singleton and performs reasonably well. Dizygotic (non-identical) twins are genetically distinct, and if only one is affected, the abnormal signal is diluted by the unaffected twin's DNA. Detection rates for trisomy 21 in twins are reported in the mid-to-high 90s, still good, but measurably below the singleton figure.
What this means in practice
NIPT is a reasonable screening choice in twins, but it should sit alongside, not replace, detailed ultrasound. The 11 to 13 week scan with nuchal translucency measured separately for each twin remains important, precisely because ultrasound can assess each baby individually. The 18 to 22 week anomaly scan is essential.
If NIPT returns high risk, the next step is a discussion about amniocentesis or CVS, sampled from each sac separately so the affected twin can be identified. This needs an experienced fetal medicine specialist.
Questions worth asking before you test
- Does this laboratory validate its test specifically for twin pregnancies? Not all do.
- What is the reported failure rate for twins, and is a repeat included?
- Has chorionicity been established on ultrasound, and has the laboratory been told?
- Is genetic counselling included, before as well as after the result?
Twin pregnancies need closer monitoring overall — for growth discordance, twin-to-twin transfusion and preterm birth. Screening for chromosomal conditions is one part of a broader plan that should be set up with a fetal medicine unit early.