The vast majority of babies are healthy, but about 1 in 100 are born with a serious mental or physical handicap. One of the most common conditions is Down Syndrome (Trisomy 21). It is more common in older mothers, but can occur in a mother of any age. Many options are available to the pregnant patient to evaluate her risk for chromosomal abnormalities.
Amniocentesis & CVS
The only way to be certain whether the fetus has a chromosomal disorder (such as Down Syndrome) is by doing a diagnostic test, such as amniocentesis or chorionic villus sampling (CVS). A CVS is performed between 11 and 14 weeks and involves taking a small amount of tissue from the placenta; an amniocentesis involves taking fluid from around the baby and is usually done after 15 weeks. Both provide a sample that contains tissue that has the same genetic make-up as the baby and will allow the baby’s chromosomes to be looked at in detail. The problem with these invasive tests is that they can cause miscarriage, even if the baby is entirely normal. The risk of miscarriage is about 1%.
A nuchal scan is an ultrasound performed between 11 and 14 weeks of pregnancy, during which fluid at the back of the baby’s neck (the nuchal translucency) is measured. All babies will have some fluid here, but in babies with Down Syndrome the fluid tends to be increased.
First trimester serum biochemistry
A blood test taken from the mother can be used to measure the levels of two hormones (free B-hCG and PAPP-A). In a pregnancy affected by Down Syndrome there is a tendency for the levels of free B-hCG to be increased and PAPP-A to be decreased.
Combined first trimester screening
Results from the nuchal scan and first trimester bloodwork can be combined, resulting in the most effective testing strategy. Combined first trimester screening picks up approximately 90% of the babies with Down Syndrome. By comparison, screening by maternal age alone picks up about 30% of the babies with Down Syndrome, screening by nuchal translucency alone picks up about 80% of the babies with Down Syndrome, and screening by first trimester bloodwork alone picks up about 60% of the babies with Down Syndrome. This testing can not diagnose a chromosomal problem, but it can be used to help you to make an informed choice about whether to have an invasive test.
The decision making process over which tests to have performed (or to have any testing performed at all) is a purely personal one. Your provider will review your options with you at the time of your first obstetrical visit, at which time you can decide on a plan of care together.