Have you ever left your OB-GYN’s office completely confused by all the genetic testing she has offered? You are not alone. Technology is advancing at a rapid pace, and there are many (often expensive) tests available to women during pregnancy. I hope to help you navigate through these options, to understand which tests may be of benefit to you and your unborn baby, and which tests are not necessary.
If you know that there is a particular disease that runs in your family, there are a number of specific tests that can be obtained prior to becoming pregnant to see if you are at risk to have an affected baby. Diseases that can be commonly tested for in this way include neurofibromatosis, thalassemia, sickle cell anemia, Tay-Sachs disease, cystic fibrosis, Duchenne muscular dystrophy, and hemophilia.
Some doctors also offer a test called expanded carrier screening, which tests for hundreds of genetic diseases, but I have found this test often causes more worry than help. It tests for so many different disorders that it is likely to find at least some abnormal genes and can cause unnecessary worry for parents. In the worst case scenario, if a couple found there was a very high risk of transmitting a disorder to their future children, they might opt to forgo pregnancy, and perhaps adopt. This is called “prenatal” testing.
Once a woman is pregnant, there are other tests that can detect various birth defects. These are called “antenatal” tests. A warning is in order here: Just because your OB-GYN mentions a test is available, do not feel that you are required to have that test. Many of these tests are very expensive and often are not covered by insurance, or the cost may be applied to your deductible, so that you are responsible for the cost. In low-risk women (women who are below age 35 and who do not have a family history of genetic disorders), many of these tests are more likely to give a false positive result (indicating there might be a problem when there is none), than a true positive result (actually detecting a problem). So, the test result may cause you to be falsely worried for no reason.
Every pregnant woman will have some routine prenatal labs that are required (checking for blood type, anemia, certain types of infections, and immunity) at her initial OB visit, and will have testing for diabetes, anemia, and infections again between 24 to 28 weeks gestation. But there are some other optional genetic tests that your doctor may mention. I’ll briefly discuss some of the common tests to help you understand what sort of problems they might be able to detect.
Cystic Fibrosis Carrier Screening
This is recommended by the American College of Obstetricians and Gynecologists, but I am not sure why. Essentially, it checks to see if you carry the gene for CF, which is a serious pulmonary disorder. If you do, then the baby’s father will need to be tested. If you both carry the gene, then there is a one in four risk that the baby will be affected. However, to determine that, you would need to have an amniocentesis, in which a needle is introduced through your belly into the amniotic fluid to withdraw cells for testing. All babies are tested for CF once they are born, so I do not feel it is necessary to go through all this prior to delivery, and I rarely recommend this test in my clinical practice.
This is a useful test to detect a variety of problems. It is a transvaginal ultrasound that measures the skin fold at the back of the baby’s neck. This is combined with a fingerstick blood test, and the lab is able to calculate a risk number for certain birth defects. For example, it may feel the risk of a certain chromosomal abnormality is 1:200, or 1:10,000. A low-risk number would certainly provide a degree of reassurance. A high-risk number does not necessarily mean the baby has a problem, but your doctor might recommend additional testing, such as a targeted ultrasound or an amniocentesis, to find out for sure. This test is performed between 11 to 14 weeks gestation and is usually combined with another blood test after 15 weeks called a maternal serum alpha-fetoprotein (MSAFP).
An additional way to get the information generated by the Ultra-Screen/MSAFP is to do a blood test after 15 weeks called a quadruple screen. The tests can also be ordered in a slightly different order, called a sequential screen.
Almost every woman receives an anatomic ultrasound between 18 to 20 weeks, which looks very carefully at the anatomy of the baby. Brain, heart, organs, spine, and limbs can be carefully assessed by this test. Information is also obtained about the amniotic fluid, placental location, length of the cervix, and even organs outside the uterus, like the ovaries. I think this test offers the most “bang for the buck,” and about half of my patients will opt to have this as the only screening test they receive in their pregnancies. Oh, yes, it will also tell you the gender, if you want to know!
There is one last screening test your doctor may offer, the cell-free DNA test. This is a very popular test and used to be offered quite frequently, because it allows a woman to find out the gender of her baby very early on (around 10 to 12 weeks). This is a blood test that is able to isolate fetal cells in the mother’s blood stream and determine the chromosomal number. Unfortunately, it is very expensive (I have seen patients receive bills ranging from $700 to $2,000), so I would not recommend it unless you are at very high risk for an abnormal chromosomal number. You will appreciate having that extra money in only two months when you find out the gender by sonogram and can then use it to buy baby clothes—or maternity clothes now!
By Ingrid Skop, M.D.