Prenatal diagnosis means diagnosis before birth. It is a way for your doctor to see if your developing baby has a problem. The two main methods are amniocentesis and chorionic villus sampling (CVS). These tests help find genetic disorders before birth.
Some parents have increased risk of having a baby with a genetic disorder or other problem. They may want to have one of these tests. Knowing about problems before the baby is born may help parents. They may be able to make better decisions about health care for their infant. Certain problems can be treated before the baby is born. Other problems may need special treatment right after delivery. In some cases, parents may decide not to continue the pregnancy.
Path to improved health
Amniocentesis or CVS is done when there is an increased risk that the baby may have genetic disorders or birth defects. It is often done if:
- You are 35 years of age or older by the time your baby is due. You have an increased risk of having a baby with a chromosome abnormality. This could include Down syndrome.
- You had a screening test that showed there could be a problem.
- You have had a child with Down syndrome or another disorder. This could include spina bifida.
- You or your partner is a known carrier of a genetic disorder, such as cystic fibrosis.
Before either procedure, you may have counseling with a genetic expert. This allows you to learn about the conditions the test may find. You will have a better idea of what those mean for you and your baby.
How is amniocentesis performed?
In this procedure, a sample of amniotic fluid (fluid around the baby) is removed from your uterus. A doctor inserts a long, thin needle through your abdomen into your uterus. He or she withdraws a small amount of fluid. The fluid is sent to a laboratory for evaluation. In the lab, the fluid can be tested for:
- genetic abnormalities
- signs of lung development.
Your body will make more fluid to replace the fluid that is taken out. The baby will not be hurt during the procedure. Some women feel mild cramping during or after the procedure. Your doctor may tell you to rest on the day of the test. Usually you can resume normal activity the next day.
How is CVS performed?
CVS removes a small sample of placenta tissue from the uterus. The sample is then sent to the lab for testing. The sample can be taken 2 ways:
- A catheter (thin tube) through the vagina. The catheter is inserted into the vagina. It is passed through the cervix and into the uterus. The doctor uses images from an ultrasound to guide the catheter to the best spot for sampling.
- A needle through the abdomen. The sample can also be obtained by inserting a needle into the abdomen. The needle then withdraws placenta tissue. Again, ultrasound is used to guide the needle.
Local anesthesia is used for this test to reduce pain and discomfort. Most women feel fine after the test. Some may have mild bleeding (spotting) afterward.
When are the tests performed?
Amniocentesis is usually performed during the 15th week of pregnancy or later. CVS is usually performed between the 10th and 12th weeks of pregnancy.
Is one test better than the other?
The main benefit of CVS is that it can be done earlier in the pregnancy. It is very accurate in detecting genetic abnormalities. But it does not detect some things that amniocentesis does. These include:
- neural tube defects (involving the spinal column or brain)
- birth defects
- Rh incompatibility (when the mother has Rh-negative blood and the baby has Rh-positive blood).
Amniocentesis might be the best option if:
- You previously had a baby with a neural tube defect.
- You or your partner have a neural tube defect.
- The results of other tests during your pregnancy have been abnormal. This could include a blood screening test done in early pregnancy.
CVS may be better if you and your doctor want to know the test results during your first trimester.
Things to consider
Amniocentesis and CVS carry a few small risks. These include:
- infection or injury to the baby
- leaking of amniotic fluid
- vaginal bleeding.
The risks of CVS are slightly higher than for amniocentesis. Your doctor will talk to you about the risks and benefits of amniocentesis and CVS.
Questions to ask your doctor
- Am I at increased risk of having a baby with birth defects or genetic abnormalities?
- Do I need to get an amniocentesis or CVS?
- Which one would be better for me?
- What is the risk of miscarriage for each procedure?
- What are the complications?
- Will the procedure hurt?
- Is bleeding normal after the procedure?
Copyright © American Academy of Family Physicians
This information provides a general overview and may not apply to everyone. Talk to your family doctor to find out if this information applies to you and to get more information on this subject.