Prenatal Genetics

What is prenatal genetic testing?

Prenatal genetic testing gives parents information about the genetic health of their fetus. There are two broad categories: screening and diagnostic.

Screening tests tell you if you are at an increased risk for having a child with genetic or chromosomal abnormalities. They are not definitive. Prenatal genetic screening tests are specific to a given pregnancy — results could be different with every pregnancy. Carrier screening tests are specific to you as an individual.

Diagnostic tests tell you whether your child does or does not have certain genetic or chromosomal abnormalities. These tests are done on cells from the fetus or from the placenta, and include amniocentesis and chorionic villus sampling (CVS). Diagnostic testing can also be done through pre-implantation genetic diagnosis (PGD), in combination with IVF.

Understanding screening test results

A negative prenatal screening result indicates a low risk that the fetus has chromosomal abnormalities. A positive result means the fetus is at a higher risk — but neither result is considered definitive or diagnostic. Non-Invasive Prenatal Screening (NIPS) is a relatively new screening test that is more accurate in some ways than older screening tests, but it still comes with its own set of limitations.

Amniocentesis and CVS

What is amniocentesis?

Amniocentesis is performed by using an ultrasound to guide a thin needle into the amniotic sac to obtain a sample of amniotic fluid. DNA testing can be done on the fetal skin cells in the fluid, and can diagnose conditions like Trisomy 21 (Down syndrome). Amniocentesis is typically performed between 15 and 20 weeks of pregnancy. There is a very small chance of pregnancy loss associated with the procedure (approximately 0.3%).

What is chorionic villus sampling (CVS)?

CVS involves taking a tissue sample from the placenta using a needle guided by ultrasound. The advantage of CVS is that it can be performed much earlier in pregnancy — as early as 10 to 13 weeks. The cells being tested are from the placenta rather than from the fetus, and while results are usually the same, they can occasionally differ. When the procedure is performed by an experienced physician, pregnancy loss risk is generally considered very low (approximately 0.22–1%).

Carrier screening

Carrier screening is a type of genetic test that determines if you carry genetic changes associated with specific conditions. We each have two copies of (almost) every gene. If a person has one working copy and one mutated copy, they are considered a carrier. Carriers are typically healthy with no symptoms.

As it becomes increasingly common to test for more genetic conditions as part of carrier screening, it is also increasingly common for individuals to find out that they carry mutations associated with one or more genetic conditions. This is not alarming — we all carry mutations.

If both parents are carriers for the same autosomal recessive condition, there is a 25% chance their child will be affected, a 50% chance the child will be a carrier like the parents, and a 25% chance the child will be an unaffected non-carrier. The probability that both partners are carriers for the same condition depends significantly on how common the condition is and on ethnic background.

What about X-linked conditions?

Females have two X chromosomes and males have one X and one Y. If a female carries a mutation in a gene on the X chromosome, she is a carrier and typically healthy. If a male has a mutation in a gene on the X chromosome, he will be affected since he has no second copy of that gene. If a female carrier has children, there is a 50% chance any child will inherit the altered gene — boys who inherit it will be affected, girls who inherit it will be carriers.

Pre-implantation genetic diagnosis (PGD)

PGD and in vitro fertilization (IVF) provide options for parents who find that one or both of them are carriers for a hereditary condition. Rather than testing a fetus during pregnancy, PGD allows genetic testing of embryos during the IVF process and before implantation. Embryos that do not carry the mutation can then be implanted. PGD is possible anytime the specific mutation related to the known genetic condition has previously been identified.