> With this test. woman's blood sample is taken after 10 weeks of pregnancy. The test measures the small fragments of fetal DNA in the mother's blood, and can determine the chance of a chromosome problem for chromosomes 21, 18, 13 and the sex chromosomes. The results are usually available within two to three weeks.

Cell-free DNA screening is a test that can determine if a woman has a higher chance of having a fetus with Down syndrome (trisomy 21), trisomy 18, trisomy 13 or an abnormality in the sex chromosomes (X and Y chromosomes).

No. Cell-free DNA screening can detect more than 99 percent of Down syndrome pregnancies and 97 percent of trisomy 18 and 95 percent for trisomy 13. Hence its considered a non invasive screening test and not as a confirmatory test.

Women who are found to be at increased risk of the above chromosomal abnormality will be advised to undergo invasive confirmatory testing which can the the Chorionic villous sampling (CVS) or Amniocentesis depending on the weeks of pregnancy. The CVS involves taking a small tissue of the placenta of the baby and amniocentesis involves taking the fluid around the baby.

CVS is carried around 11-13 weeks of pregnancy and Amniocentesis after 15 weeks. Even though these tests are confirmatory they are not offered to all patients as they are invasive tests and carry an additional 1% risk of miscarriage.

We recommend cell-free DNA screening to women who have a higher than average chance of having a fetus with Down syndrome. This includes women whose age is 35 years and more, those who, and those who have had a previous pregnancy or baby with Down syndrome or trisomy 13 or 18.

Women with a lower probability of Down syndrome can also have cell-free DNA screening as its more sensitive than the routine tests offered for screening of these chromosomes.

Cell-free DNA screening is not as accurate in women with twins, and cannot be used in women with triplets.