Prenatal Screening and Myths

I have been learning a lot about the different types of prenatal screening and diagnostic tools available to Canadian women lately and many of the topics featured corrected a lot of myths I used to believe. So I am writing this post to help eliminate some of the widely held beliefs about screening and tests.

First of all I will talk about the different types of screens available:
1. IPS (Integrated Prenatal Testing): What is it? Two blood tests (at 11-13 weeks and 15-16 weeks) plus an ultrasound. What does it test for? Down Syndrome, Trisomy 18, and neural tube defects. Pro: high detection rate for Down Syndrome. Con: 2 tests and results received later than other methods

2. FTS (First Trimester Screening): What is it? Blood test (11-13 weeks) plus an ultrasound. What does it test for? Down Syndrome. Pro: earlier results than IPS. Con: Does not check for neural tube defects.

3. MSS (Maternal Serum Screening): What is it? Blood test (16 weeks plus). What does it test for? Down Syndrome, trisomy 18 and neural tube defects. Pro: Can be used if IPS and FTS were not done. Con: Lower detection rate for Down Syndrome.

MYTH: A good prenatal screen means the baby will be perfect
TRUTH: Prenatal screening is not perfect and 2-3% of all babies have a major abnormality at birth.

MYTH: A positive screen means the baby definitely has a defect
TRUTH: A positive result means the baby is at a higher risk of having a defect, but special diagnostic tests are still needed. The false positive rate of screening tests is about 5% which means that 5% of positive results are wrong.

MYTH: Prenatal testing is just for people who would have an abortion
TRUTH: Prenatal testing can be good for couples because if a defect is found then parents can prepare for a special needs child, and doctors can prepare for the best way to plan the birth and neonatal management once the child is born

After prenatal testing, if a positive result is obtained there are a few more options for couples who want diagnostic testing to determine what is potentially wrong with the child.

Types of Diagnostic Tests:
1. Amniocentesis: This is performed around 15-18 weeks. A needle is used to remove about 20 mL of fluid from the amniotic sac surrounding the baby. This can be used to determine if the baby has chromosome abnormalities (Down syndrome), neural tube defects, blood type, blood disorders (like sickle cell anemia), and infection. Risks: The biggest risk is miscarriage which is 0.5% or 1 in 200 pregnancies. (Note: background risk at this time is 2%)

2. Chorionic Villus Sampling (CVS): A catheter is used to obtain cells samples from the placenta. This can test for chromosome abnormalities, single gene defects (like cycstic fibrosis), and gender. It is usually performed between 10-12 weeks. There is a 1% risk of miscarriage or 1 in 100 pregnancies. (Note: background miscarriage risk is also a bit higher earlier in the pregnancy and risk depends greatly on the physician who is performing the procedure)

I learned a lot while researching all the different prenatal options women have today. I was surprised how inexact some testing is, and how despite the best efforts of medical professionals, not all birth defects can be detected. But I think the key with any medical decisions people have to make is the information they receive. I did not realize how common birth defects actually are, and I think that is how the majority of other people think as well. Knowing the limits of testing is important for setting expectations, both for yourself and patients.