The fetal sex DNA test and NIPT are often mentioned together as ways to find out a baby's sex during pregnancy, but the two tests differ in purpose and scope. The fetal sex DNA test analyses fetal DNA for the presence or absence of the Y chromosome and confirms sex only, while NIPT is a health screening test that looks at chromosomal abnormalities as well as sex.
The fetal sex DNA test can be carried out from 6 weeks of pregnancy onward and its accuracy is about 99%, while NIPT is carried out at around 10 weeks of pregnancy to check for abnormalities such as Down syndrome. It is important to know that both are screening tests, not diagnostic tests.
What does the fetal sex DNA test check?
For a long time, fetal sex was confirmed by reading an ultrasound scan at around 14 weeks of pregnancy, but depending on the baby's position the reading could be unclear or the sex could turn out to be the opposite. Amniocentesis, in which a needle is inserted into the uterus, is fairly accurate, but it is invasive and a considerable burden. The fetal sex DNA test was introduced to make up for these limits by confirming sex with a simple blood draw.
DNA from the fetus circulates in the mother's blood, and the test works by analysing whether the Y chromosome is present in it. If the Y chromosome is detected the baby is judged to be a boy, and if it is not detected, a girl. However, when the test is done very early in pregnancy or in a multiple pregnancy, it is difficult to tell the sex of each baby apart, so the accuracy is not complete.
- Timing of the test — very early in pregnancy the amount of fetal DNA may not be sufficient, so accuracy can be lower
- Multiple pregnancy — with twins, the presence or absence of the chromosome can be confirmed, but it is difficult to tell the sex of each baby
- How the result is read — if the Y chromosome is detected the baby is judged to be a boy, and if not, a girl
How is NIPT different from the fetal sex DNA test?
NIPT and the fetal sex DNA test both use the mother's blood, but the range of what they check is clearly different. NIPT is regarded as a method with high sensitivity and specificity, and it checks broadly for chromosomal abnormalities in the fetus as well as sex, which helps medical care to begin quickly when it is needed.
A high-risk result on NIPT does not mean that an abnormality is confirmed straight away, and the result should be taken as an indicator of risk. When a high-risk finding is seen, the usual procedure is to confirm it again with a further test such as chorionic villus sampling or amniocentesis, and then decide how to proceed.
- Purpose — it checks for chromosomal abnormalities such as Down syndrome and Edwards syndrome together with sex
- Timing — it can be carried out from around 10 weeks of pregnancy
- Reading the result — a high-risk result means a level of risk, not a diagnosis, and is confirmed again with a further test
Which of the two tests should you choose?
It is most accurate to understand the two tests not as substitutes for each other but as separate tests with different purposes. For a woman who simply wants to know the sex early, the fetal sex DNA test after 6 weeks of pregnancy is suitable. For a woman who also wants to check for chromosomal abnormalities or the level of health risk, NIPT at around 10 weeks of pregnancy is the more suitable option.
When choosing, it should be kept in mind that both are screening tests rather than diagnostic tests, and the result needs to be interpreted together with the course of the pregnancy rather than trusted blindly. Beyond sex and chromosome testing, an ultrasound assessment of the baby's growth and checks on the mother's health, such as for pre-eclampsia or gestational diabetes, should also be carried out in each trimester.
