Why it's done

By Mayo Clinic Staff

First trimester screening is done to evaluate your risk of carrying a baby with Down syndrome. The test also provides information about the risk of trisomy 18.

Down syndrome causes lifelong impairments in mental and social development, as well as various physical concerns. Trisomy 18 causes more severe delays and is often fatal by age 1.

First trimester screening doesn't evaluate the risk of neural tube defects, such as spina bifida.

Because first trimester screening can be done earlier than most other prenatal screening tests, you'll have the results early in your pregnancy. This will give you more time to make decisions about further diagnostic tests, the course of the pregnancy, medical treatment and management during and after delivery. If your baby is diagnosed with Down syndrome, you'll also have more time to prepare for the possibility of caring for a child who has special needs.

Other screening tests can be done later in pregnancy. An example is the quad screen, a blood test that's typically done between weeks 15 and 20 of pregnancy. The quad screen can evaluate your risk of carrying a baby with Down syndrome or trisomy 18, as well as neural tube defects, such as spina bifida. Some health care providers choose to combine the results of first trimester screening with the quad screen. This is called integrated screening. This can improve the detection rate of Down syndrome.

Remember, first trimester screening is optional. Test results only indicate whether you have an increased risk of carrying a baby with Down syndrome or trisomy 18, not whether your baby actually has one of these conditions.

Before the screening, think about what the results will mean to you. Consider whether the screening will be worth any anxiety it might cause, or whether you'll manage your pregnancy differently depending on the results. You might also consider what level of risk would be enough for you to choose a more invasive follow-up test.

Oct. 10, 2015