First trimester screening

First trimester screening is a prenatal test that offers early information about a baby's risk of certain chromosomal conditions — Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18).

First trimester screening has two steps:

  • A blood test to measure levels of two pregnancy-specific substances in the mother's blood — pregnancy-associated plasma protein-A and human chorionic gonadotropin
  • An ultrasound exam to measure the size of the clear space in the tissue at the back of the baby's neck (nuchal translucency)

Typically, first trimester screening is done between weeks 11 and 14 of pregnancy — although a form of the screening can be done as early as nine weeks.

Using your age and the results of the blood test and the ultrasound, your health care provider can gauge your risk of carrying a baby who has Down syndrome or Edwards syndrome.

If your risk level is low, first trimester screening can offer reassurance of a healthy pregnancy.

If your risk level is moderate or high, you might choose to follow first trimester screening with another test that's more definitive.


Why it's done Risks How you prepare What you can expect Results

First trimester screening is done to evaluate your risk of carrying a baby who has Down syndrome. The test also provides information about the risk of Edwards syndrome.

Down syndrome causes lifelong impairments in mental and social development, as well as various physical concerns. Edwards syndrome 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, medical treatment and the course of the pregnancy. 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 who has Down syndrome or Edwards syndrome, 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 — although you won't learn the final results until both tests are completed and analyzed.

Remember, first trimester screening is optional. Test results only indicate whether you have an increased risk of carrying a baby who has Down syndrome or Edwards syndrome, 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 handle 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.

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