Third Trimester: Amniocentesis
The test is most often offered between 15 and 20 weeks of gestation (usually around week 16) to a pregnant woman under any of the following circumstances:
- She is 35 years of age or older.
- There is a strong family history of a genetic disorder from the parents.
- Both parents possess a gene for an inherited disorder.
- The level of maternal alpha-fetoprotein (AFP) is either lower or higher than expected.
- A previous child in the family had a birth defect or chromosome problem.
Amniocentesis may also be performed after 32 weeks of gestation to help assess the degree of lung development for babies at risk of premature delivery.
During the procedure, a needle is inserted through the walls of the abdomen and uterus and into the thin-walled sac of fluid that surrounds the developing fetus. A small amount of amniotic fluid is withdrawn. This fluid contains AFP produced by the baby and fetal cells. The fetal cells can be tested for chromosomal or genetic abnormalities. A gene analysis may be performed based on family history (i.e., another child born with a defect or with a hemoglobinopathy) or on the results of screening tests performed on the parents (such as for cystic fibrosis). Approximately two weeks are needed to complete the testing.
There is a slight risk with amniocentesis that the needle inserted into the amniotic sac may puncture the baby, cause a small amount of amniotic fluid leakage, cause an infection, and in rare cases cause a miscarriage.