Genetic screening in early pregnancy
By Dr. Elizabeth Oler MD/OB/GYN
When you’re pregnant, there are a lot of things to think about and many of them can cause a great deal of anxiety. Making the right decisions for you and your baby is difficult, and we are here to help! One of the decisions you may make early on is whether or not to do genetic screening. If you search the internet, you’ll hear a lot of different advice, a lot of which is misleading. We are here to give you the facts so that you can make the right decision for your pregnancy – for genetic screening and beyond!
First, let’s talk about screening tests. We use these in many different conditions to detect disease. A screening test is the first step to finding out if a condition is present. Screening tests can’t diagnose something - they can only tell us if we are at risk. So if you do have a positive screening test, further testing is always required to give you a definitive answer.
Genetic screening in pregnancy is used to screen for three different chromosomal conditions - trisomy 13, trisomy 18, and trisomy 21. These conditions are not inherited from mom and dad like other genetic conditions are, but they happen randomly during the development of eggs and sperm. Trisomy 13 and 18 are rare, and lead to very severe conditions that are not compatible with life for the baby. They will always be detected on your anatomy ultrasound. Trisomy 21 is also known as Down Syndrome, which has highly variable features, some of which cannot be detected by your anatomy ultrasound. All of these conditions are rare, but trisomy 21 is more common than the other two.
There are two tests we offer to screen for these conditions. One, the NIPT, or noninvasive prenatal test, can be done as early as 10 weeks. The other, the quad screen, is done between 16 and 22 weeks. They are nearly equivalent in how accurate they are, but both tests can have false positives or false negatives. Let’s break down the pros and cons of each type of test, so you can pick the right one for you.
The NIPT detects the baby’s chromosomes, or DNA, which are floating in your bloodstream, and come from the placenta. From the fragments of baby’s placental DNA, we can detect if there are abnormal levels of chromosomes. Even though it looks at the DNA, this test cannot pick up other conditions – but that might happen in the future. The main positive of this test is that because we are looking at baby’s DNA, we can find out if the baby is a boy or girl early. One drawback of this test is that the accuracy is lower when you are younger, but it improves as you get older, and the ideal time to use this test is in your 30s. The other much more important drawback is how expensive it is! Insurance often won’t cover this test if you’re younger than 35 at the time of your due date. Without insurance coverage, the cost of this test ranges from hundreds to thousands of dollars.
The quad screen, or maternal serum screen 4, is a little different. This test looks for levels of proteins in your bloodstream that are associated with the pregnancy. With the combination of your blood levels of these proteins, we can calculate your risk of having one of the three trisomies. The main benefit of this test is that it includes levels that can tell you other things about the health of your pregnancy and your placenta. One part of this test, the PAPP-A, can be decreased with babies that will end up having growth or placenta abnormalities.
The quad screen also includes a level called alpha fetoprotein (AFP) that can tell you if the baby is at risk for any defects in the skin, such as open neural tube defects (spina bifida) or abdominal wall defects such as gastroschisis. These can often be detected on ultrasound, but small ones don’t always show up. AFP can be done on its own too, so we often recommend getting the AFP separately with the NIPT.
The other big benefit of the quad screen is that it is covered by insurance and is a lot less expensive.
So now that you are armed with all of this information, how do you decide if you even want these tests at all? When I talk to patients about genetic screening, I always make sure to let them know that this is up to them – these tests are not mandatory. Finding out about these conditions can be helpful, but for many families, they feel better not knowing and would prefer to find out either at the anatomy ultrasound or when the baby is born. Tests can be done after the anatomy ultrasound if we find something abnormal, so it is perfectly ok to wait. It is also ok to get these tests early so that you can have the information and so that you can be prepared, and make decisions about the course of your pregnancy that are right for you.
Remember, no test is perfect and we have false negatives and false positives. They are rare, and so are all of these conditions. Please talk to us if you want to learn more about these tests or if you have questions about this complex information. It is normal to be confused and we are here to help!