Early on in your pregnancy your health care provider may talk to you about prenatal testing. The idea behind a prenatal test is to detect risks, birth defects, and serious diseases in your developing baby. For many the tests are scary, and because you do not know what your options are, and what you should do, or even what to ask your doctor about them, you are left in a bit of a lurch. The following is a guide to prenatal testing:
First, let's look at the tests. What do they test for? What are the most common prenatal testing options?
The following are the most common tests:
- Urine tests: These tests check for urinary tract infections, and signs of gestational diabetes and preeclampsia.
- Cervical and vaginal tests: these check for cervical cancer, Chlamydia, and gonorrhea. You will also get a Group B strep test at about week 34.
- Glucose screening test: This is done during weeks 24-30, and it checks for gestational diabetes.
- Blood tests: These tests are done to determine your blood type, Rh factos, whether or not you are anemic, and to see if you have, or are at risk for rubella, chicken pox, syphilis, or hepatitis. You may also be tested for HIV, as it can be passed on to your baby.
What are the most common prenatal tests?
Cystic Fibrosis carrier testing:
- This is a prenatal test that is done to find out if your baby is at risk for having Cystic Fibrosis. This genetic disorder can seriously shorted life, and requires daily treatment, so knowing if you are having a child with CF can help you prepare for it mentally and physically.
- How do they test for CF? This is typically done by blood testing, but can also be done using a sample of saliva. You can get this test when you are pregnant, but it can also be done when you are not expecting. If you do get it when pregnant, try to get it done during the first trimester so that you and your doctor will have more time to prepare for it if you are a carrier.
- This test is done to see if you are a carrier, not to see if your baby actually has it or not. If you and the father both are carriers, then you have a 25% chance of having a baby with CF. If you find that you both are carriers, your doctor will probably talk to you about other testing options to see if your baby actually has it or not.
- There are no risks to this test, and it will not hurt your pregnancy at all. If you find that you are both carriers of CF you may want to do further testing, which can be more risky.
Maternal serum screening:
- This is a test that measures certain chemical markers in your blood while you are pregnant, and uses that combined with your ultrasound to determine if your risk is high or low for having a baby with a birth defect. There are a lot of different ways this test can be performed. They are listed below.
- First it is important to note that these screenings are to be done during the first and second trimester.
- First trimester screening should be done between 10 and 13 weeks. In this screening you will get an ultrasound and a blood test. Your ultrasound will be used to make sure the gestational age of the baby is what is anticipated, and also to measure the fluid on the back of the baby's neck. The ultrasound combined with the blood test help doctors estimate the chances that your baby will have Down syndrome. Once again, this is not a test that says whether or not your baby actually has the disorder, just how high the chances are.
- Second trimester screening is done between weeks 15 and 22, and is often referred to as a quad screening. This I a blood test that measures the levels of four different markers in your blood. Your health care provider may choose to combine this blood test with an ultrasound. This screening test, like the previously mentioned one just estimates the risk of having a baby with Down syndrome. It also estimates risks for trisomy 18, and NTD. In addition these tests can often detect problems with the placenta, and help you make more informed decisions about whether or not you feel the need for further testing.
- You can also opt for a combination of both first and second trimester screenings which is called integrated screening. The results of this screening are from blood drawn between 10 to 13 weeks, and the ultrasound you get at that time, as well as the blood drawn at 15 to 22 weeks. This is the most accurate form of screening for estimating the risks of Down syndrome, trisomy 18, and NTDs, as well as for problems with the placenta.
- You can also opt for sequential screening that basically means you do the first trimester screen, and then if you are placed at high risk, you go ahead with other testing without waiting for the second trimester test results.
- Maternal serum screening poses no risks to your pregnancy, and basically are there to set your mind at ease, or help you prepare for high risk pregnancy. It helps you determine your needs for further testing.
- When you get a maternal serum screen done your results are either reported at screen positive, or screen negative for a particular birth defect. Screen positive means that you are in the higher risk category for having a child with a birth defect, again it is important to note this is a risk factor test, it does not mean you will actually have a child with a birth defect, it just means you are at higher risk than the average pregnant person. A screen negative test means you are in the lower risk category.
- If you screen positive then your doctor should talk to you about your options and help you find the best plan for you. This may mean doing the CVS tests later, or it may mean CVS testing and amniocentesis:
- These are both highly accurate tests for genetic disorders. They are invasive tests because they require the entrance into the womb to get test samples, but they can tell you fairly certainly whether or not your baby has a genetic disorder.
Let's take a look at these two tests separately, and see what they tell you and what the risks are:
- CVS: This test is done between 10 and 12 weeks, and is done by taking a sample of the placenta. It is used to check for specific genetic disorders, not just the risk of having them. The disorders it is used to detect include: Down syndrome, trisomy 18, and cystic fibrosis. Your doctor will use earlier tests to determine what conditions specifically to use a CVS test to test for.
- The way this test is administered is a doctor will insert a hollow needle into the placenta, either by going through your abdomen, or by going up through the vagina and cervix. Your doctor will use an ultrasound to help guide the needle. They will withdraw a small sample of the placenta, and then take the needle out.
- The sample of placenta is sent to a lab for testing. In about 10 days you get your results. This test is said to be 99% accurate.
- There are some risks with this, but rarely enough to cause a miscarriage. In fact, miscarriage happens less than 1% of the time. However, for most women it does cause some cramping, and or bleeding, and can cause infection.
- Amniocentesis: This test is done between 15 and 20 weeks, and for this test a sample of amniotic fluid is taken from around the baby. This test is most often used to check for neural tube defects and genetic disorders like Down syndrome and trisomy 18. It can also be used to test for CF.
- The way this test is administered is a doctor inserts a hollow needle through your abdomen into your uterus, using an ultrasound to help guide them. The doctor then withdraws a small sample of amniotic fluid, and removes the needle. Again this sample is sent to a lab. In 10 to 14 days you hear back, again with 99% accuracy.
- The risks are generally the same, although there is a slightly smaller chance of miscarriage for the amniocentesis then for CVS.
- If you are over 35, have a child with a birth defect or disorder, or have other tests that show risks for these, your doctor will likely recommend a CVS or amniocentesis. Your health care provider will help you determine which test is best for you, and if your results come back abnormal, they will help you learn as much as you can, and be as prepared as possible for a child with a genetic disorder.