What is Amniocentesis and Why Would You Need This Procedure?

From your first prenatal visit, the testing begins. You should expect a few tests for every state of pregnancy. One of the tests your doctor may suggest is amniocentesis.  

What Is Amniocentesis? 

Amniocentesis is an invasive prenatal procedure in which a small amount of amniotic fluid is extracted from your baby’s amniotic fluid to check your baby for chromosomal disorders. Amniocentesis is usually done between your 15th and 20th week of pregnancy.  

When you undergo amniocentesis, a 4-inch, 22-gauge needle will be pushed through your abdominal wall, through your uterus, chorion, and amniotic sack into the amniotic fluid. You are likely to feel discomfort, but most women liken the pain to menstrual cramps. 

When I was a resident, patients were sent for an ultrasound to determine the safest place to insert the needle. Back then, those doing ultrasounds were reluctant to allow physicians to perform procedures in their areas. The patient would be returned to the obstetrical department with a X on the place the ultrasound showed to be the safest place to insert the needle without injuring the baby or the placenta. 

I was on good terms with the ultrasound tech at that time. She was not put off by my doing the ultrasound in the radiology department. Today amniocentesis is done live in the ultrasound room, as I had done all those years ago.  

In special cases such as with twins, amniocentesis can be done for many reasons, usually to check on the health of both babies. With twins, you need to know which amniotic sack you are sampling, and if you want to check on both twins, you will need to take samples from the amniotic sac of both babies. 

Why Would You Need an Amniocentesis?  

Amniocentesis can show if your baby will have Tay-Sachs disease, Down’s syndrome, sickle cell disease, or cystic fibrosis. Later in your pregnancy, amniocentesis may also be used to determine fetal lung function, to identify possible fetal lung infections, or the presence of too much amniotic fluid (polyhydramnios).  

Amniocentesis is often done for women over 35 who are looking for age-related anomalies.  

What are the risks with amniocentesis? 

There is always the risk of hitting the cord, placenta, or fetus during an amniocentesis. Ultrasound can trigger labor, most often by hitting the cord or placenta, although I’ve never personally seen this happen. However, doing the amniocentesis with live ultrasound markedly decreases these risks. 

In residency there was one fetal death after an amniocentesis for fetal lung maturity. It wasn’t my patient, but it was a good lesson. There is no such thing as an amniocentesis without risk. Today there are other ways to look for lung maturity like the first ultrasound for dates. 

While amniocentesis is usually done between your 15th and 20th week of pregnancy. Amniocentesis can be used at a later date to check for fetal lung maturity. However, when the test is conducted at a later stage of pregnancy, the risk of complications increases.  

Alternative to Amniocentesis 

There are alternatives to amniocentesis. Chorionic villus sampling (CVS) can be done, usually before 13 weeks. The chorion is the sac that contains the amniotic sac. That is, you baby is in an amniotic sac which is within another sac called the chorion 

CVS still requires a needle to extract the chorionic fluid for testing, but this approach avoids penetrating the amniotic sac, so lessens the risk to the baby. 

Biparietal diameter (BPD) with ultrasound is another option for looking for fetal maturity. 

Talk with Your Physician About Your Concerns with Amniocentesis 

Perhaps you will not have any need for an amniocentesis in your pregnancy. If you think you have a medical history which may indicate the need for amniocentesis, by all mean, talk with your doctor about this test and what it involves.  

One thing we know. Anxiety in pregnancy causes many kinds of problems. If you are worried about amniocentesis and whether you might need one, talk with your doctor. 

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Dr. Alan Lindemann

Obstetrician-Gynecologist (OB/GYN)​

He is an obstetrician and maternal mortality expert with 4 decades of medical practice beginning in Minnesota and presently in North Dakota. He has delivered around 6,000 babies with zero maternal deaths.