What are typical genetic tests during pregnancy? Who should do genetic testing?

Expert:  Yael Hoffman Sage, MD, MPH, an obstetrician at Brigham and Women’s Hospital.

Genetic tests during pregnancy

There are a wide variety of genetic tests during pregnancy. These include:

• Screening tests such as maternal serum screening (e.g. First Look or Sequential Screen) or cell-free DNA. Doctors use to determine whether your baby is at increased risk for chromosomal conditions such as Down syndrome, Edwards syndrome and Turner’s syndrome.
• Ultrasounds can also be used to both identify birth defects and assess risk for genetic abnormalities.
• Maternal blood work and ultrasound are both screening tools for chromosomal abnormalities, but neither is 100 percent diagnostic of a genetic problem.

Additionally, genetic carrier testing is a way to determine whether you are at increased risk of having a baby with an inherited genetic condition. Doctors advise genetic testing if you or your partner has:

• a family history of birth defects (such as club foot or cleft lip)
• genetic conditions (such as cystic fibrosis or sickle cell disease).
• if you are a member of a group predisposed to certain diseases. Some ethnicities and/or countries of origin are associated with a genetic predisposition for certain diseases. For example, people of Ashkenazi Jewish descent are more likely to be carriers for Tay-Sachs disease.

Genetic counselors can help you and your partner decide which inherited diseases and conditions to screen for both before and during your pregnancy.

Genetic testing during pregnancy using amniocentesis or chronic villus sampling

Your doctor may suggest diagnostic testing by either amniocentesis (sampling of amniotic fluid) or chronic villus sampling (sampling of the placenta) in the following cases:

• an ultrasound detects an abnormality
• your blood work shows a high risk for a chromosome abnormality
• you are a carrier of a known genetic abnormality such as cystic fibrosis or Tay Sachs,

Amniocentesis

Doctors typically order an amniocentesis between 16 to 23 weeks,  is done by inserting a very thin needle into the abdomen to reach the amniotic sac. Once the needle is in, the clinician will be able to sample the amniotic fluid. Amniotic fluid contains cells from the fetus with genetic information that can be tested. If you’re carrying more than one fetus, your doctor will need to obtain fluid from each amniotic sac. In cases of abnormal blood tests or abnormal ultrasound, amniocentesis is an extremely useful test, though it comes with a very small increased risk of miscarriage or infection.

Chronic villus sampling

Chronic villus sampling (CVS) samples the placental tissue. This tissue typically contains genetic material from the fetus. CVS is performed during the first trimester, between 10 and 13 weeks. Doctors perform CVS in the following cases:

• when blood work shows a high risk for chromosome abnormalities
• there is an ultrasound abnormality of the fetus
• if the patient is a known carrier for a genetic defect.

To extract tissue from the placenta, a doctor or nurse will insert either a catheter with a syringe on one end into the vagina and guide it through the cervix. In some cases, a needle is placed into the placenta through the abdomen. Using ultrasound to see where the needle is, your health professional can a obtain a sample from the placenta.