What is amniocentesis?

Amniocentesis is a procedure performed on pregnant women in their second trimester to diagnose or rule out birth defects. The procedure is usually performed in the doctor’s office by an obstetrician. With the help of ultrasound, a thin needle is inserted through the abdomen into the uterus to remove a small amount of amniotic fluid (the fluid surrounding the baby). The amniotic fluid is then sent to a lab for testing and the results are usually returned to your doctor within 2 weeks.

Who needs amniocentesis?

Not all pregnant women have an amniocentesis. It is offered when it has been determined that there is an increased risk of chromosomal or genetic birth defects. The increased risk is determined by the obstetrician and is due to one of many reasons:

• Maternal age. Studies have shown that chromosomal birth defects increase as the age of the mother increases. It is common for obstetricians to suggest amniocentesis if the mother will be 35 or older at the time of delivery. The most common disorder detected due to older maternal age is Down syndrome. Current data has shown that women in their twenties have a 1 in 1250 chance of delivering a child with Down syndrome whereas a woman who is 35 has a 1 in 385 chance.
• History. This includes the personal genetic history of the parents and their families as well as the genetic history of other children by that couple. Any prior history of genetic disorders within a family can prompt an obstetrician to suggest further testing.
• Other tests. Other testing such as an AFP (alpha-fetoprotein) or CVS (chorionic villus testing) can have results that indicate that further testing is needed. For example, the maternal serum AFP test combined with estriol and hCG is a screening test for neural tube defects. The test may come back abnormal for many reasons and therefore an amniocentesis is performed to rule out birth defects.

Are there risks?

As with any procedure, there are potential risks. The most serious risk is the possibility of miscarriage occurring after the procedure. Current data indicates that only 1 in 200 cases result in miscarriage and these numbers are continually improving with the use of better ultrasound equipment. It is also not known how many miscarriages would have occurred regardless of the procedure or not.

Following an amniocentesis the patient may resume normal activities. However, strenous exercise, such as running, should be avoided for several days. Complications are rare, but some women experience cramps similar to those during menses. If these cramps persist for several hours or several days following amniocentesis, a reduction in general activity is recommended, but not complete bed rest. A physician should be contacted if either bleeding or leakage of fluid from the vagina occurs.

Remember: No prenatal test or ultrasound can guarantee a perfect baby. Some birth defects cannot be determined prenatally. However, more than 95% of women in the high-risk category receive reassuring results from their prenatal testing.

If you have any questions about amniocentesis or birth defects, we will be glad to discuss your questions with you.

Thomas G. Stovall, M.D.
Dr. Stovall is a Clinical Professor of Obstetrics and Gynecology at the University of Tennessee Health Science Center in Memphis, Tennessee and Partner of Women’s Health Specialists, Inc.