Chromosome abnormalities occur in about 1 in 150 live births. Their risk rises with maternal age, but they can affect pregnancies at any age.
Birth defects in India
Trisomy 21 (Down syndrome) is the most common autosomal aneuploidy in live births, at about 1 in 700 worldwide. India has nearly half a million babies born with malformations each year; Down syndrome accounts for about 21,000 of these, or roughly 1 in 1,150 births. A study of 94,610 newborns at centres in Mumbai, Delhi and Baroda informs these figures.
Factors associated with chromosome abnormalities
- Advanced maternal age (over 35 years)
- Parental translocation or other chromosomal abnormality
- Structural abnormalities seen on prenatal ultrasound
- A screen-positive double, triple or quadruple marker test
Who should be offered screening?
According to ACOG Practice Bulletin (October 2020), screening and diagnostic options should be offered to all pregnant patients, regardless of age or risk. Every patient has the right to pursue or decline prenatal genetic screening and diagnostic testing, based on accurate information and personal values.
Screening options
The most common approach in India is single time-point screening:
1First trimester
Nuchal translucency (NT) scan with serum double marker test.
2Second trimester
Triple or quadruple (quad) marker test.
3NIPT
Cell-free fetal DNA screening, increasingly popular as prices fall, with very high accuracy for Down syndrome.
4Combined screening
Integrated, sequential and contingent screening using both trimesters (less common in India).
Screening tests compared
| Test | Gestational age | T21 detection | False positive | Key advantage | Key disadvantage | Sample | Cost (INR) | Turnaround |
|---|---|---|---|---|---|---|---|---|
| NIPT (cell-free fetal DNA) | 9–10 weeks onward | 99% | 2–4% | Highest detection rate; early option | Can be affected by maternal aneuploidy or disease | Maternal blood (Streck tube) | 15,000 | 7–10 days |
| Double marker / first-trimester screen | 10–13⁶⁄₇ weeks | 82–87% | 5% | Early, single time point | Lower detection; needs NT scan | Maternal serum (plain red) | 2,400 | 24–48 hours |
| Quadruple marker | 15–22 weeks | 81% | 5% | Single time point | Lower detection than combined tests | Maternal serum (plain red) | 3,800 | 24–48 hours |
| Integrated screen | 10–13⁶⁄₇ then 15–22 weeks | 96% | 5% | High detection rate | Two samples; no first-trimester result; needs NT | Maternal serum (plain red) | 10,000–12,000 | 24–48 hours |
Screening tests estimate risk; they do not diagnose. A high-risk result should be confirmed with prenatal diagnosis (CVS or amniocentesis) after genetic counselling.
References
- Nussbaum RL, McInnes RR, Willard HF. Thompson & Thompson Genetics in Medicine, 8th ed. Elsevier, 2016.
- Verma IC. Indian Journal of Pediatrics, 2000;67(12):893–8.
- ACOG Practice Bulletin, Screening for Fetal Chromosomal Abnormalities, October 2020.