Although most women in America give birth to normal, healthy babies, many are still concerned about the possibility of birth defects. Some tests that are performed during pregnancy can detect birth defects before the baby is born. Two of these tests include amniocentesis and chorionic villus sampling (CVS). This page gives information about these two tests and the birth defects they detect.

What is a birth defect?

A birth defect is characterized by a serious change in a baby’s normal appearance or organ function. Around 3% of all babies will be born with a major birth defect. Sometimes, babies can be born with more than one birth defect.

There are several different things that cause birth defects. Sometimes, a mutation in the genes of the parents is passed on to the child, making it inherited. Sometimes, a baby can become deformed from being exposed to certain drugs, chemicals, and substances (like alcohol) during pregnancy. Sadly, sometimes birth defects are random without any cause.

What are genetic birth defects?

In each cell in your body, there is a special compartment where your genes (genetic material) are housed. Your genes are made up of 23 chromosomes, which each have two copies- one from your mother, and one from your father. These were used to create you by combining your father’s sperm with your mother’s egg. Each chromosome has lots of DNA (genetic material) inside of it. This helps determine what traits your body will display, such as hair color, eye color, height, etc. The genes also determine how your organs will work and determine the growth of a fetus. Therefore, a single error in a chromosome could result in a birth defect.

A birth defect can be passed on to a baby by inheriting one or two malfunctioning genes from a parent. Sometimes, birth defects can even occur when neither parent has displayed any signs of a defect. Some other disorders, such as Down’s syndrome, are caused by problems with the number of chromosomes that are present in the baby’s DNA.

Certain defects can be found in one of two ways:

• Amniocentesis: removing a small sample of amniotic fluid that surrounds the fetus for testing
• CVS: sampling the villus structures that make up the placenta.

Some of the disorders CVS and amniocentesis are used to find include Tay-Sachs, Down’s syndrome, cystic fibrosis, and sickle-cell disease. It can also be used to determine the baby’s sex.

Who should be tested?

Although genetic testing is not required during pregnancy, it is encouraged to some women who may be at a higher risk for birth defects:

• Couples who have a family history of birth defects
• Couples who already have a child with a birth defect
• Pregnant women aged 35 or older
• Pregnant women with abnormal genetic test results

It is also important to remember that sometimes, the tests don’t test for every single genetic disorder. This means that sometimes, your baby may still have a defect, that couldn’t be detected from testing.

What is amniocentesis?

Amniocentesis is a simple procedure that is commonly performed around 16-18 weeks of pregnancy. 

• Procedure: Amniocentesis is a simple procedure where a needle is used to sample some of the amniotic fluid around the fetus.
  • First, the patient lies down with her abdomen exposed. An ultrasound is used to determine the best placement for the needle. 
  • The needle is then guided through the uterus into the amniotic sac. 
  • A small, one ounce sample is extracted and the needle is withdrawn. 
  • Then, the sample is sent to a lab where the cells are grown in cultures for testing.
• Results: Usually, it takes around two weeks for the cultures to be ready to test. The scientists have to check the chromosomal structure as well as their number. They also have to check to make sure there aren’t too many substances such as alpha-fetoprotein in the amniotic sac. 
• Risks: Amniocentesis is a fairly safe test. However, some side effects may include:
  • Cramping
  • Infection
  • Bleeding
  • Leaking of amniotic fluid
  • Miscarriage

What is chorionic villus sampling?

CVS is a procedure that can be done earlier in the pregnancy than amniocentesis. It’s done at around 10-12 weeks from conception, which allows for earlier detection. However, it isn’t as widely practiced as amniocentesis, so you may have to search for a doctor that performs them.

• Procedure: During CVS, a sample of placental cells are taken to be studied.
  • There are two ways to collect these: through the vagina, or through the abdomen. 
    • If collected from the vagina, a speculum is first inserted, and a plastic tube is guided up to the placenta, where a small sample is removed.
    • If collected from the abdomen, a slender needle is inserted into the abdomen and through the uterus to the placenta. It is very similar to amniocentesis.
  • The cells are then sent to a lab. There, they are grown in cultures and tested.
• Results: CVS tests for many of the same defects as an amniocentesis. However, it can’t test for neural tube defects. If you are concerned about this, it could be beneficial to also schedule a blood AFP (16 -20 weeks) test to check out whether or not your baby will have them. 
• Risks: CVS carries a slightly more advanced risk of miscarriage, because it is performed earlier in the pregnancy. Sometimes, limb defects or infection can occur after CVS.

What are the advantages of each method?

Amniocentesis is much more common. So, it is easier to find a place that performs them. It also carries a lower risk of miscarriage than CVS, and can test for neural tube defects.

However, CVS can be performed earlier in the pregnancy, which is beneficial, because it allows for more time to make a decision after potentially abnormal test results. Some women may want to keep the option of abortion open if they find that their pregnancy will have birth defect.

In conclusion

Amniocentesis and CVS are excellent ways to determine whether or not your child may have birth defects. Both have advantages and disadvantages. Talk to your doctor if you are at all concerned about birth defects, and about your choices in your pregnancy.