What is an amniocentesis?

An amniocentesis is a procedure performed to obtain a small sample of the amniotic fluid that surrounds the fetus to diagnose genetic disorders, open neural tube defects (ONTDs) such as spina bifida, and/or fetal infections such as cytomegalovirus (CMV).  

An amniocentesis is generally offered between the 15th and 20th week of pregnancy, but can be performed any time after the amniotic and chorionic membranes have fused. Any pregnant person could decide to have an amniocentesis, however, there are some common indications, such as:

• Being at an increased risk for chromosome abnormalities, such as women who are over 35 years of age at delivery
• Having an abnormal result on maternal serum screening, indicating an increased risk for a chromosomal abnormality or neural tube defect
• Having a family history of a genetic disorder  
• Identification of a fetal anomaly, such as a structural heart defect, detected during ultrasound

How is an amniocentesis performed?

An amniocentesis is a procedure that involves inserting a long, thin needle through the mother's abdomen into the amniotic sac to withdraw a small sample of the amniotic fluid for examination. The amniotic fluid contains cells shed by the fetus, which contain genetic information. Although specific details of each procedure vary slightly, generally, an amniocentesis follows this process:

• The woman's abdomen is cleansed with an antiseptic
• The doctor may or may not give a local anesthetic to numb the skin
• Ultrasound is used to help guide a hollow needle into the amniotic sac
• A small sample of fluid is withdrawn for laboratory analysis
• Strenuous activities should be avoided for 24 hours following an amniocentesis
• Women may feel some cramping during or after the amniocentesis

Women with twins or other multiples may need sampling from each amniotic sac, in order to study each baby. Depending on the position of the baby, placenta, amount of amniotic fluid, or mother's anatomy, sometimes the amniocentesis cannot be performed.

Women who are Rh negative will likely receive an Rh(D) immune globulin shot following amniocentesis.  

The fluid is sent to a genetics laboratory so that the cells can be analyzed. Alpha-fetoprotein, a protein made by the fetus that is present in the fluid, may also be measured to rule out an open neural tube defect, such as spina bifida. Results are usually available in about 10 days to two weeks, depending on the laboratory.

What are the risks and benefits of amniocentesis?

After an amniocentesis, women may experience cramping, bleeding, or leaking of amniotic fluid. There is also a slight risk of infection. The risk of miscarriage is generally considered to be less than 1 percent after an amniocentesis in the second trimester of pregnancy. This is only slightly higher than the normal risk of miscarriage without an amniocentesis at this time in pregnancy.

Testing performed on amniotic fluid is diagnostic, meaning an amniocentesis can help confirm a tentative diagnosis of an abnormality found with other testing/screening. It may also find that a fetus does not have the abnormality/genetic disorder that was suspected. This allows couples to plan the remainder of pregnancy and to consider their options. Amniocentesis offers:

• Diagnosis of a chromosomal abnormality, such as Down syndrome (trisomy 21)
• Diagnosis of open neural tube defects
• Diagnosis of an infection, such as CMV
• Diagnosis of other genetic disorders or conditions related to abnormalities noted on ultrasound