Amniocentesis is done for different reasons at different stages of pregnancy.
Genetic amniocentesis can provide information about your baby's genetic makeup. Generally, genetic amniocentesis is offered when the test results might have a significant impact on the management of the pregnancy — or your desire to continue the pregnancy. Typically, the value of this information can only be assessed by you and your partner.
Genetic amniocentesis is usually done after week 15 of pregnancy. Rarely, genetic amniocentesis might be done as early as week 11 of pregnancy.
You might consider genetic amniocentesis if:
- You had positive results from a prenatal screening test. If the results of a screening test — such as the first trimester screen or noninvasive prenatal testing — are positive or worrisome, you might opt for amniocentesis to confirm or rule out a diagnosis.
- You had a chromosomal condition or a neural tube defect in a previous pregnancy. If a previous pregnancy was affected by Down syndrome or a neural tube defect — a serious condition affecting the brain or spinal cord — this pregnancy is at higher risk, too.
- You're 35 or older. Babies born to women 35 and older have a higher risk of chromosomal conditions, such as Down syndrome.
- You have a family history of a specific genetic condition, or you or your partner is a known carrier of a genetic condition. In addition to identifying Down syndrome and spina bifida, amniocentesis can be used to diagnose many other conditions — such as cystic fibrosis.
Maturity amniocentesis can determine whether a baby's lungs are ready for birth. This type of amniocentesis is done only if early delivery — either through induction or C-section — is being considered to prevent pregnancy complications for the mother. It's usually done between 32 and 39 weeks of pregnancy. Earlier than 32 weeks, a baby's lungs are unlikely to be fully developed.
Other reasons for amniocentesis
Occasionally amniocentesis is used to:
- Evaluate a baby for infection or other illness
- Decrease the volume of amniotic fluid
- Evaluate the severity of anemia in babies who have Rh sensitization — an uncommon condition in which a mother's immune system produces antibodies against a specific protein on the surface of the baby's blood cells
Your health care provider might caution against amniocentesis if you have:
- Placental problems, including the placenta partially or totally covering the cervix (placenta previa) or the placenta peeling away from the inner wall of the uterus before delivery (placental abruption)
- A history of premature labor before 34 weeks of pregnancy
- An incompetent cervix — a condition that occurs when weak cervical tissue causes or contributes to premature birth or the loss of an otherwise healthy pregnancy