Amniocentesis is a procedure in which a small sample of the amniotic fluid is taken from the amniotic sac. The amniotic sac is the fluid-filled bag that surrounds the fetus in the uterus of a pregnant woman. It has the same genetic makeup as the fetus.
Who is a candidate for the procedure?
A woman may wish to have an amniocentesis done if:
she will be 35 years old or older when her baby is born
she or her partner has had a child or fetus with a chromosomal abnormality
she or the father has a family history of a serious chromosomal abnormality
the baby might have a serious problem passed on to one gender only, such as muscular dystrophy
she wants to find out if her baby has a serious health problem that can be detected by examining its genes
she has had two or more miscarriages
An amniocentesis may also be offered to a woman who has an abnormal alpha fetoprotein blood test result. A high level of AFP may indicate a neural tube defect, such as spina bifida. A low level can indicate the presence of Down syndrome.
Amniocentesis can detect many disorders in a fetus, including:
cystic fibrosis, a condition in which abnormal body secretions cause problems in the lungs and digestive system
Down syndrome, which causes mental retardation and health problems
neural tube defects, such as spina bifida
phenylketonuria, or PKU, which interferes with the body's ability to handle certain food components
sickle cell anemia, a condition in which abnormal red blood cells interfere with many body processes
Tay-Sachs disease, which causes blindness, loss of function, and early death
Amniocentesis is usually performed 15 to 18 weeks after the woman has missed a period. A chorionic villus sampling is a similar procedure that involves taking a sample of the fluid in the sac surrounding the fetus. It is done several weeks earlier in the pregnancy.
Amniocentesis is occasionally done in the third trimester, toward the end of the pregnancy. It may be done at this time for the following reasons:
to see if the baby's lungs are mature enough for delivery
to diagnose uterine infections, especially if the woman's membranes have ruptured early
to measure any anemia in the baby caused by Rh incompatibility
to see if the baby needs blood transfusions
How is the procedure performed?
Shortly before the procedure, the woman will be asked to fill her bladder. The full bladder helps the healthcare provider see the pelvic organs with a pregnancy ultrasound. A local anesthetic is used to numb a small area on the woman's abdomen. The provider uses the ultrasound to guide the insertion of a thin needle. A small amount of amniotic fluid is withdrawn and sent to the lab for testing.