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Why it's done
By Mayo Clinic staffAmniocentesis is done for different reasons at different stages of pregnancy.
Genetic amniocentesis
Genetic amniocentesis can provide information about your baby's genetic makeup. Generally, genetic amniocentesis is offered when the test results may 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15 of pregnancy, when the two layers of fetal membranes have fused enough that a sample of amniotic fluid can be safely withdrawn. Rarely, genetic amniocentesis may be done as early as week 11 of pregnancy.
You may consider genetic amniocentesis if:
- You had abnormal results from a prenatal screening test. If the results of a screening test — such as the first trimester screen — are positive or worrisome, you may opt for amniocentesis to confirm or rule out a diagnosis.
- You had a chromosomal abnormality 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 abnormality of the brain or spinal cord — this pregnancy is at higher risk, too.
- You're age 35 or older. Babies born to women age 35 and older have a higher risk of chromosomal abnormalities, such as Down syndrome.
- You have a family history of a specific genetic disorder, or you or your partner is a known carrier of a genetic disorder. In addition to identifying Down syndrome and spina bifida, amniocentesis can be used to diagnose many other conditions — such as cystic fibrosis. This requires specialized testing of the amniotic fluid, however, so there must be a reason to test for these conditions.
Maturity amniocentesis
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
- Diagnose a uterine infection
- Evaluate the severity of anemia in babies who have Rh disease — 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
- Ghidini, A. Amniocentesis: Technique and complications. http://www.uptodate.com/home/index.html. Accessed Jan. 18, 2010.
- Eddleman K, et al. Pregnancy loss rates after midtrimester amniocentesis. Obstetrics & Gynecology. 2006;108:1067.
- Diagnosing birth defects. American Congress of Obstetrics and Gynecology. http://www.acog.org/publications/patient_education/bp164.cfm. Accessed Jan. 18, 2010.
- Cunningham FG, et al. Williams Obstetrics. 23rd edition. New York, N.Y.: McGraw-Hill Companies; 2010. http://www.accessmedicine.com/content.aspx?aID=6021591. Accessed Feb. 8, 2010.
- Simpson JL, et al. Prenatal genetic diagnosis. In: Gabbe SG, et al. Obstetrics: Normal and Problem Pregnancies. 5th ed. Philadelphia, Pa.: Churchill Livingstone Elsevier; 2007:1.
- Amniocentesis. March of Dimes. http://www.marchofdimes.com/professionals/14332_1164.asp. Accessed Feb. 10, 2010.
- Cystic fibrosis: Prenatal screening and diagnosis. American Congress of Obstetricians and Gynecologists. http://www.acog.org/publications/patient_education/bp171.cfm. Accessed Feb. 9, 2010.
- Gillen-Goldstein J, et al. Assessment of fetal lung maturity. www.uptodate.com/home/index.html. Accessed Feb. 10, 2010.
- American Congress of Obstetricians and Gynecologists. Invasive prenatal testing for aneuploidy. ACOG Practice Bulletin No. 88. Obstetrics and Gynecology. 2007;110:1459.
- Harms RW (expert opinion). Mayo Clinic, Rochester, Minn. Accessed Feb. 15, 2010.
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