The two main types of prenatal genetic testing are tests that detect chromosome conditions, such as Down syndrome, and tests that determine whether someone is a carrier of a genetic condition, such as cystic fibrosis.

Testing for Chromosome Conditions

Testing for chromosome conditions can be done through a screening test or a diagnostic test.

• A screening test is non-invasive, meaning only a maternal blood sample and, in some cases, an ultrasound is required to perform the testing. Though often very accurate, screening tests can have false positives and false negatives. Screening tests for chromosome conditions include first trimester screening, second trimester screening, and non-invasive prenatal testing (also known as NIPT, NIPS, or cell-free DNA screening).
• A diagnostic test involves using a needle to sample part of the placenta or the fluid that surrounds the baby (called amniotic fluid), and diagnostic tests carry a small chance for a pregnancy complication such as infection or pregnancy loss. This chance is typically less than 1%. Diagnostic tests are the most definitive form of prenatal testing available. Diagnostic testing performed in the first trimester is called chorionic villus sampling (CVS) and diagnostic testing in the second trimester is called amniocentesis.

Testing for Carrier Status

Carrier screening analyzes a person’s DNA to see if they are a carrier for a genetic condition. Carriers of genetic conditions are typically healthy and often do not know they are carriers. If both parents are carriers of the same genetic condition, or in some cases if just the mother is a carrier, then there would be an increased chance for their baby to be affected. Carrier screening can be limited to genetic conditions that are more common based on a person’s ethnicity or can be broader to include conditions affecting a wide array of ethnicities. Although carrier screening can identify individuals at risk to have a child with a genetic condition, it does not screen for all possible genetic conditions and does not assess risks for chromosome conditions, birth defects, or intellectual disabilities. Ideally, carrier screening is conducted prior to pregnancy, but screening can also be done at any time during pregnancy. Screening can be done using a saliva sample, buccal swab or blood sample and can be conducted on one reproductive partner at a time, or both simultaneously.

First trimester screening reports the chance for specific chromosome conditions (Down syndrome, trisomy 18, and sometimes trisomy 13) to be present in a pregnancy. Done between 11 and 13 weeks gestation, the measurement of two pregnancy-related hormones obtained from a maternal blood sample are combined with two ultrasound measurements to calculate these risks. When results indicate an increased risk, additional evaluations such as ultrasounds and diagnostic tests are offered.

Second trimester screening (also called maternal serum screening) measures levels of biologic chemicals in a pregnant woman’s bloodstream. Using a blood sample drawn between 15-20 weeks of a pregnancy, this screening reports the chance for specific chromosome conditions (Down syndrome and trisomy 18), as well as neural tube defects like spina bifida, to be present in a pregnancy. Results are usually available within a week. When results indicate an increased risk, additional evaluations such as ultrasounds and diagnostic tests are offered.

Prenatal genetic testing is optional and the decision to have this testing should be based on your own beliefs, personality, and needs. Some people want to know if there is an increased chance to have a pregnancy affected with a genetic condition so that they can use that information to plan for the future. Other people want to know because they might not continue a pregnancy if it were found to be affected with a genetic condition. Some people do not want to have prenatal genetic testing because they feel that the results would not change their pregnancy plans and could make them more anxious.

The accuracy of prenatal genetic testing for chromosome conditions varies depending on the screen or test that was performed.

• For example, first or second trimester screening tests can detect as many as 96% to as few as 64% of pregnancies affected with Down syndrome, depending on the specific test that is performed. Non-invasive prenatal testing/cfDNA testing can detect over 99% of pregnancies affected with Down syndrome. The detection rates for other chromosome conditions are lower because those conditions are more rare.
• False positives can occur. The chance for a positive first or second trimester screening or non-invasive prenatal testing/cfDNA result to be a false positive depends on the specific screen performed, the condition it screened positive for, and other factors such as the maternal age.
• CVS and amniocentesis are the most definitive tests available. While no test can be 100% correct every time, these tests give you as much certainty as possible.

Carrier screening accuracy will depend on the technology used and how common the genetic condition is. Many carrier screening panels can detect over 99% of people who are carriers of the conditions included on the panel.

First trimester screening is performed using a maternal blood sample and a first trimester ultrasound called the nuchal translucency (NT). The NT ultrasound involves taking a measurement along the back of the baby’s neck. The NT measurement and the blood sample are used together to determine the results of first trimester screening.

Second trimester screening, non-invasive prenatal testing/cfDNA, and carrier screening are all performed with only a blood sample. Carrier screening can be performed on either biological parent, but all other testing listed here must be performed on the pregnant patient. Carrier screening can also be performed on a saliva sample or buccal swab.

Prenatal diagnostic testing, such as chorionic villus sampling (CVS) and amniocentesis, are considered more invasive testing options. CVS involves using a needle to obtain a sample of the placenta. Amniocentesis involves using a needle to obtain a sample of the fluid that surrounds the baby (called amniotic fluid). Both of these procedures carry a risk to the pregnancy, such as risk of infection or pregnancy loss, but those complications occur less than 1% of the time.