Pregnancy is a really chaotic yet joyous time. From the first time you see the two pink stripes on that pregnancy test, to the seemingly endless nausea and vomiting, then the first time you see its little heartbeat on the ultrasound, and finally the sigh of relief when you hear it cry for the first time. But we know that not all pregnancies are smooth sailing and that the formation of a full-grown baby from two tiny cells is no easy feat.
When the egg and sperm first meet they must replicate and exchange genetic information with each other via chromosomes, which contain DNA. During the course of this exchange and replication may be errors that result in certain syndromes. The three most common are Down Syndrome, Edward Syndrome and Patau Syndrome. They are also known as trisomy 21, 18, and 13 respectively. The introduction of non-invasive prenatal testing (NIPT) allows us to detect such syndromes without risk to the unborn baby.
During pregnancy, the unborn baby releases some of its DNA into the maternal blood. By taking a sample of the mother’s blood we can pick out the baby’s DNA and analyze it for chromosomal abnormalities. Currently, various laboratories do the test, all with more than 99% accuracy and a false positive rate of 0.1-0.4%1. Despite its high detection rate, it is not 100% as it is only a screening test. This means that even though it is positive, it cannot be certain that your baby is affected. In most situations, if the test is positive your doctor will discuss with you further test options to confirm the diagnosis. The confirmatory tests are either chorionic villi sampling (CVS) or amniocentesis. The choice of either test will depend on how advanced your pregnancy is, the expertise available and your choice. However, the tests are considered invasive tests and carry a miscarriage rate of 0.5 to 1%.
Prior to the introduction of NIPT, women were testing for the three syndromes using first trimester combined screening (FTS). This screening calculates the baby’s risk of having any of the syndromes using a combination of baby’s neck fold thickness, maternal age, background risk and specific pregnancy proteins (e.g. beta-hCG, PAPP-A). This method has a detection rate of approximately 90%2. Although NIPT is superior to FTS in its detection rate, both are still only screening tests.
In addition to testing for abnormalities of chromosome 21,18, and 13, NIPT is also able to detect the baby’s sex and look for sex chromosome abnormalities. The exception is when the pregnancy is a twin or higher order pregnancy. In the case of identical twins, the laboratory will assume both babies contribute equally to the DNA sample. However, in non-identical twins or vanished twins, this cannot be assumed and hence the test will not be able to proceed.
NIPT is a wonderful technology that provides us with non-invasive options to determine the baby’s well being. Please discuss with your doctor whether this test is right for you.
Dr. Zara L. Chan
Dr. Zara Chan is a Specialist in Obstetrics and Gynaecology and works at OT&P Healthcare’s Woman and Child Clinic in Central as well as the Wanchai Clinic. For more information about Dr Chan and about OT&P, please go to www.otandp.com
- Nicolaides KH, Syngelaki A, Ashoor G, et al. Noninvasive prenatal testing for fetal trisomies in a routinely screened first-trimester population. Am J Obstet Gynecol 2012;207:374.e1-6.
- MM Gil, MS Quezada, R Revello et al., Analysis of cell-free DNA in maternal blood in screening for fetal aneuploides: updated meta- analysis, Ultrasound in Obstetrics & Gynecology, 2015, 45(3):249-266