New options for genetic testing in pregancy

December 18, 2013

By Ashley Fuller, MD

Congratulations! You just found out you are pregnant and so many things start going through your mind. When you’re not dry heaving or completely exhausted you start planning for your exciting future but in the back of your mind you wonder… how do I know everything is okay with my baby?

We are entering an exciting time in the field of obstetrics that involves less invasive and more accurate options for genetic testing in pregnancy. ACOG, the American College of Obstetrics and Gynecology, recommends that all women, regardless of maternal age, be offered prenatal testing for chromosomal abnormalities.

For quite sometime our options for this testing have been somewhat stagnant. We have offered noninvasive risk profiling that involves a mixture of blood tests and ultrasounds at various times in the first and second trimesters to help evaluate the baby’s risk for Down syndrome or other lethal chromosomal abnormalities. Depending on how these tests are processed, the sensitivity ranges from 80-95% with about a 5% false positive rate. They are fairly accurate at identifying babies at higher risk, but can have false positive results (meaning an abnormal result followed by more invasive testing that shows normal results but of course this causes a lot of worry for the patient).

Obviously we want to be able to offer testing that has a high rate of detection and a low rate of false positives. More invasive testing is often offered also. This testing involves removing a sample of placental cells called chorionic villus sampling, or removing a sample of fetal cells from the amniotic fluid called amniocentesis. These cells are then analyzed for chromosomal abnormalities. Although these invasive tests are the most accurate, they do carry a small risk of miscarriage or fetal loss.

Fortunately, new testing has come out on the market called cell free fetal DNA testing. This is a screening test that takes a sample of maternal blood anytime after 10 weeks, removes the maternal cells and analyzes the remaining free DNA. About 3-13% of the free DNA is from the fetus, specifically the placenta. Evaluation of this DNA can accurately detect Down syndrome (trisomy 21) and lethal chromosome abnormalities like trisomy 13 and 18. In high-risk women, this test has a 98% sensitivity and only a 0.5% false positive rate. Not only is it incredibly accurate, but it also looks for a Y chromosome—telling you the fetal sex as early as 10 weeks!!

Although this is very exciting new technology, there are a couple caveats. These tests are approved and recommended for women who are high risk to have chromosomal abnormalities.

How do you know if you’re high risk? Generally, age is the biggest factor. Women over 35 have a higher risk of chromosomal abnormalities in their pregnancies than younger women. Other women that are high risk are those with an abnormal fetal ultrasound, a history of prior pregnancy with chromosomal abnormality, or another prenatal screening test with abnormal results. Also this test, although very accurate, is still a screening test, not a diagnostic test. This means that if you have an abnormal result you would still need an invasive test like an amniocentesis or chorionic villus sampling to verify the results.

As our technology advances there is a definite possibility that this testing will be offered to all pregnant women, regardless of age or risk. Hopefully as our technology advances we will be able to test for more diseases through this noninvasive technique. If you are pregnant or are thinking of getting pregnant, consider talking to your doctor about what they recommend for genetic testing options.