Learning About Birth Defect and Genetic Testing During Pregnancy

What is birth defect and genetic testing?

Testing can be done during pregnancy to look for possible genetic conditions or birth defects in a fetus. These problems may have only a minor impact on a child's life. Or they can have a major effect on quality or length of life.

You and your doctor or midwife can choose from many tests. You may have no tests, one test, or many tests.

What are the types of tests?

You may have a screening test, a diagnostic test, or both. Screening tests show the chance that a fetus has a certain birth defect or genetic condition, such as Down syndrome, spina bifida, or trisomy 18. Diagnostic tests show if a fetus has a certain genetic condition or birth defect.

• Screening tests and when they're done:
  • Blood tests at 10 to 13 weeks (first trimester)
  • Cell-free fetal DNA test at 10 weeks or later (first trimester)
  • Nuchal translucency test around 11 to 13 weeks (first trimester)
  • Alpha-fetoprotein (AFP) at 15 to 18 weeks (second trimester)
  • Quad screening at 15 to 22 weeks (second trimester)
  • Ultrasound at 18 to 22 weeks (second trimester)
• Diagnostic tests and when they're done:
  • Chorionic villus sampling (CVS) at 10 to 13 weeks (first trimester)
  • Amniocentesis at 15 to 20 weeks (second trimester)

Sometimes doctors or midwives will look at the combined screenings that you've had over a period of time. This is called an integrated screening.

What are the screening tests?

• Blood tests are done to look at different substances in your blood. These include tests such as cell-free fetal DNA and quadruple (quad) blood tests. The quad test checks the levels of alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), estriol (uE3), and inhibin A in your blood.
• Nuchal translucency test uses ultrasound to measure the thickness of the area at the back of the fetus's neck. An increase in thickness can be an early sign of certain genetic conditions or birth defects. Ultrasound is a tool that uses sound waves to make pictures of the fetus and placenta inside the uterus.
• Ultrasound lets your doctor or midwife see an image of the fetus. It can help your doctor look for problems of the heart, spine, belly, or other areas.

What are the diagnostic tests?

Chorionic villus sampling (CVS) looks at cells from the placenta. To do the test, your doctor may put a thin tube through your vagina and cervix to take out a small piece of the placenta. Or the doctor may take out the piece through a needle in your belly. This test can diagnose many genetic diseases. But it can't find problems with the spinal cord.

Amniocentesis looks at the amniotic fluid that surrounds the fetus. Your doctor will put a needle through your belly into your uterus and take out a very small amount of fluid to test.

What are the risks of these tests?

There is a small risk of a miscarriage after a chorionic villus sampling (CVS) or an amniocentesis. Your doctor or midwife or genetic counselor can help you understand this risk. These tests are generally very safe.