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About prenatal diagnostic tests

​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Diagnostic tests detect or rule out certain genetic disorders or birth defects. Diagnostic tests include amniocentesis, and chorionic villus sampling.

While diagnostic tests could be done in any pregnancy, many consider these tests only if there are increased risks for their pregnancy.

Diagnostic tests do carry a risk of miscarriage.

Your health care provider, together with a genetic counselor, will talk with you about the need for diagnostic testing and answer any questions you may have.

Amniocentesis

Amniocentesis is usually done after 15 weeks of pregnancy. A doctor guides a thin needle through your abdomen and uterus. A small sample of amniotic fluid is withdrawn and sent to a lab where cells from the fluid are studied to detect extra chromosomes (as in Down syndrome) or other chromosomal abnormalities.

The amniotic fluid can also be tested to check for open spina bifida.

Problems from amniocentesis are not common, but there may be some side effects, including:​

  • Cramping
  • Vaginal bleeding
  • Infection
  • Leaking amniotic fluid

There is a small chance (less than 0.5%) that amniocentesis may cause a miscarriage.

How long for results 10-14 days

What it looks for ​(​accuracy rate) All chromosome disorders (99%); Open spina bifida ​(96%)

Considerations Diagnostic; Risk of miscarriage(less than 0.5% or 1 in 200)

Chorionic villus sampling

Chorionic villus sampling checks for some of the same chromosomal problems as amniocentesis, but cannot detect open spina bifida.

The test is done at 11 to 14 weeks of pregnancy.

A doctor guides either a small tube through your vagina and cervix or a thin needle through your abdomen and uterine wall and takes a small sample of tissue from the placenta. The sample contains cells with the same genetic makeup as the fetus.

The sample is sent to a lab where the cells are studied to check for chromosomal or other defects.

Problems from chorionic villus sampling are not common, but there is a small chance that the test may cause a miscarriage.

How long for results 7-10 days

What it looks for ​(​accuracy rate) All chromosome disorders (99%)

Considerations Diagnostic; Can be done earlier than amniocentesis; Risk of miscarriage (small); Does not test for open spina bifida