When you’re pregnant, your doctor will want to do several prenatal screenings that look for certain genetic and chromosomal disorders. You’ll probably be offered one during your first trimester and another in your second trimester.
Most women get results that are within the normal range and don’t need any further tests. But if any of your screenings show signs of a problem, your doctor will offer you follow-up tests so you can find out for sure whether your growing baby has any genetic or chromosomal problems.
Your doctor will need a small sample of your baby’s cells to check his chromosomes. There’s more than one way that your doctor can get these cells. These tests are known as karyotype tests.
What Do Karyotype Tests Look For?
Karyotype tests study your baby’s chromosomes to see if they are normal or not. Humans have 46 chromosomes (23 pairs). Babies inherit 23 from their mom and 23 from their dad.
Sometimes, babies have an extra chromosome, a missing chromosome, or an abnormal chromosome. Karyotype tests will determine if any of these have happened with your baby. The most common things that doctors look for with karyotype tests include:
Down syndrome (trisomy 21). A baby has an extra, or third, chromosome 21. This affects how the baby looks and learns.
Edwards syndrome (trisomy 18). A baby has an extra 18th chromosome. Babies usually have many problems, and most don’t survive more than a year.
Which Karyotype Test Do I Need?
Your doctor will suggest which karyotype test is right for you based on how far along you are in your pregnancy and on your risks. (The tests can only be performed during certain weeks of your pregnancy.)
The tests include:
Chorionic villus sampling (CVS). Doctors use a long needle to remove a small sample of your baby’s cells from the chorionic villi, which are tissues that are found within the placenta (the organ that forms within your womb to nourish your growing baby).
These cells are sent to a lab, where they will be studied. The test results can show whether or not your baby has Down syndrome, trisomy 13, trisomy 18, or other genetic problems.
If your doctor says that you need CVS, you can have it done between 10 and 13 weeks. There is a chance of miscarriage in women who have the test -- it happens to 1 out of every 100 women -- and risk to the baby, so doctors only suggest it if there’s a good chance that your baby has a problem.
Amniocentesis. Doctors get samples of your baby’s cells by taking a small amount of amniotic fluid (the liquid that surrounds your baby in the womb) with a long needle they stick through your abdomen. Again, the cells are sent to a lab, where they will be studied.
The test results can show whether your baby has any of the genetic problems that the CVS testing can find, as well as some neural tube defects, which are severe problems that can affect your baby’s brain or spine.
If your doctor says that you need amniocentesis, you can have it done between 15 and 20 weeks. There is a chance of miscarriage, although it’s a smaller chance than with CVS. It happens to just 1 out of every 200 women who have the procedure.
What Do the Results Mean?
When the lab sends your results back, they’ve looked at your baby’s chromosomes, so the results are definite: Either your baby has a genetic problem or he does not.
This is different from the earlier screening tests, which can only say that there’s a high or low chance of a problem. You should discuss your test results with your doctor.