What is first trimester screening?

First trimester screening combines fetal ultrasound and blood tests for the mother. It’s done during the first trimester of pregnancy, during weeks 1 to 12 or 13. It can help find out the risk of the baby having certain birth defects. This includes chromosome defects, such as Down syndrome (trisomy 21), or trisomy 18 or 13.

First trimester screening may include:

• Ultrasound test for fetal nuchal translucency.  This test uses ultrasound to look at the back of the baby's neck. It checks for increased fluid or skin thickening. These might mean a defect.

• Blood tests.  The blood tests measure two substances found in the blood of all pregnant women:

  • Pregnancy-associated plasma protein-A. This protein is made by the placenta in early pregnancy. Abnormal levels could mean an increased risk for a chromosome defect.

  • Human chorionic gonadotropin. This hormone is made by the placenta in early pregnancy. Abnormal levels could mean an increased risk for a chromosome defect.

• Cell-free fetal DNA screening. This tests the baby's DNA that is in your blood. It checks for certain abnormal chromosomes. It can also check for defects in the fetal sex chromosomes (X or Y). Cell-free fetal DNA screening does not find structural birth defects, such as spina bifida or defects in the abdominal wall. 

If first trimester screening is abnormal, you may need more testing for diagnosis. This may include chorionic villus sampling, amniocentesis, or another ultrasound.

Why might I need first trimester screening?

First trimester screening can tell if the baby might have a birth defect. The screening test is usually offered to all pregnant women toward the end of the first trimester. Cell-free fetal DNA testing may be offered to women at increased risk, such as women over age 35. These screening tests may not be accurate in women with a multiple pregnancy (twins or more).

What are the risks of first trimester screening?

First trimester screening involves an ultrasound and blood tests. These are low-risk tests. But if the tests are not done at the right time during the pregnancy, the results may be wrong. For example, this might happen if your due date was miscalculated. This can cause unnecessary worry and concern for you and your partner.

First trimester screening is not 100% accurate. It’s only a screening test to see if there is an increased risk for a birth defect. It also helps to see if you need more testing or watching during your pregnancy.

False-positive results can show a problem when the baby is actually healthy. False-negative results show a normal result when the baby actually does have a health problem.

How do I get ready for a first trimester screening?

You don't need to do anything special to get ready for the ultrasound or blood tests.

What happens during a first trimester screening?

First trimester screening generally involves the following tests. They are done between weeks 10 and 14:

• Ultrasound test. Usually a transvaginal ultrasound is done to look at the baby. In this test, a small ultrasound transducer is inserted into the vagina.

• Blood test. Blood is drawn from a vein and sent to the lab for analysis. Cell-free fetal DNA blood testing may be done as early as 9 weeks. 

What happens after a first trimester screening?

After the tests are done, you will meet with your healthcare provider to review the results. First trimester screening is not 100% accurate, and does not give a diagnosis. If the results are abnormal, your healthcare provider will discuss follow-up testing for a diagnosis. If appropriate, you will be offered genetic testing and counseling.

Next steps

Before you agree to the test or procedure make sure you know:

• The name of the test or procedure

• The reason you are having the test or procedure

• What results to expect and what they mean

• The risks and benefits of the test or procedure

• What the possible side effects or complications are

• When and where you are to have the test or procedure

• Who will do the test or procedure and what that person’s qualifications are

• What would happen if you did not have the test or procedure

• Any alternative tests or procedures to think about

• When and how you will get the results

• Who to call after the test or procedure if you have questions or problems

• How much you will have to pay for the test or procedure