What is the issue?
How accurate is the new test (genomics-based non-invasive prenatal testing (gNIPT)) for detecting abnormal chromosome number in an unborn baby’s genetic material (DNA) found in the mother’s blood? We assessed the accuracy for the screening of Down syndrome (trisomy 21), Edward syndrome (trisomy 18), Patau syndrome (trisomy 13), Turner syndrome (45,X), Klinefelter syndrome (47,XXY), Triple X syndrome (47,XXX) and 47,XYY syndrome. There are different methods in use for gNIPT. We assessed MPSS (massively parallel shotgun sequencing) that tests whole DNA and TMPS (targeted massively parallel sequencing) that tests targeted DNA.
There are 46 chromosomes (23 pairs) in humans. Abnormal numbers of chromosomes can cause genetic disorders for which there are no cures. Having an extra chromosome is called trisomy and an excess (or less) of sexual chromosome is called sex chromosome abnormality (SCA). The most common trisomy is Down syndrome which occurs in about one in 1000 babies. Children with Downs have slow growth, characteristic facial features and mild to moderate intellectual disability, with some requiring specialist education later in life. However, the symptoms vary from mild to severe so that some infants lead relatively normal lives. The other trisomy or SCA conditions have varying degrees of disability but the chance of a baby being affected is much less.
Current screening tests for these conditions require confirmation if the baby has the condition or not and for this an invasive test like amniocentesis is used. Amniocentesis is where fetal cells that float in the fluid surrounding the unborn baby are collected by putting a fine needle through the mother’s abdomen and collecting the fluid. Alternatively, tissue can be collected from the placenta (chorionic villus sampling (CVS)). With these invasive tests, pregnant women are exposed to a higher chance of losing their baby even if the baby is unaffected by Down syndrome. So, this invasive test is only offered to women who are thought to have a higher chance of having an affected unborn baby
What we did
We looked for studies that included women of any age, ethnicity and gestational age who were carrying either a single baby or more than one. We searched for studies (up to July 2016) that assessed the accuracy of the new test.
What we found
We found 65 studies with a total of 86,139 pregnant women, including 3141 affected pregnancies. Forty-two studies (65%) enrolled pregnant women with a high chance of having babies with abnormal chromosome number. Forty-eight (74%) studies included only women with a singleton pregnancy. Forty-four studies (68%) used MPSS and 21 studies (32%) used TMPS.
gNIPT seems to be accurate for screening unborn babies (either singletons or twins), especially for detecting Down syndrome, trisomy 18 and trisomy 13. However, there were some problems with how the studies were conducted which makes us cautious about our findings. This may result in gNIPT appearing to perform better than it really does.
Other important information to consider
gNIPT method appears to perform well in identifying unborn babies with abnormal number of chromosomes. However, when a gNIPT detects an abnormal chromosome number, then a confirmation using invasive tests (like amniocentesis or CVS) is still needed before pregnancy-related decisions can be made.
It is important that pregnant women are given full information on the possible health problems that might arise for babies affected by an additional chromosome. For example, with Down syndrome though some children have considerable disability, others can lead relatively normal lives. In addition, in this review most studies enrolled pregnant women with increased chance of having babies with abnormal chromosome number, so our findings do not directly apply to general populations of pregnant women.