Learn More About This Procedure
What can I expect during an amniocentesis?
The expert answers
The amniotic fluid is the fluid that surrounds the baby. During an amniocentesis (amnio), a thin needle is inserted into the amniotic cavity and a small amount of amniotic fluid is withdrawn. The two most common reasons to perform an amnio are to test for genetics (earlier in pregnancy) and to test for lung maturity (later in pregnancy).
A genetic amniocentesis is usually performed from 15 to 18 weeks gestation to look for the presence of chromosomal abnormalities. Most humans have 46 (23 pairs) of chromosomes.
One of the most common chromosome disorders is Down Syndrome, or Trisomy 21. Laboratory tests are performed on the amniotic fluid to check the karyotype (a study of the number and shape of the chromosomes), alpha-fetoprotein assays (to test for neural tube defects) or enzymatic studies (to test for diseases such as Tay-Sachs and Cystic Fibrosis). The risk of having a miscarriage following a genetic amnio is typically reported as 1 in 200. In more experienced centers, it is probably less.
Before the amnio is performed, usually a thorough ultrasound is done. The placenta, uterus, fetal anatomy, size and amniotic fluid are all evaluated. Using the ultrasound, a safe pocket of fluid is identified. The skin is then cleansed, usually with alcohol and a Betadine solution. The ultrasound is then put back on, and the physician inserts the needle. During the entire procedure, the ultrasound is used to check the position of the needle. Sometimes the baby may move and the needle may have to be readjusted, using ultrasound guidance.
The needle that is used is long, but very skinny. It usually takes about a minute to get the fluid out. After the needle is removed the sonographer takes a quick look at the heartbeat again. The baby will regenerate the fluid within six hours. Most people are pleasantly surprised that the amnio was not as painful as they thought it would be. The most common description of how it feels is "a needle stick and then some mild cramping."
If you have a negative blood type, you will need a Rhogam shot after the procedure. As a precaution, for 24 hours following the amnio you should avoid any strenuous activity or sexual intercourse. You will be asked to call your doctor if you experience heavy bleeding, leaking fluid, severe cramping or a fever. Usually your results will be back within two weeks.
If you are having an amniocentesis late in pregnancy for lung maturity, the test is performed the same exact way. However, you will have your results within hours.