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Definition
By Mayo Clinic staff
First trimester screening is a prenatal test that offers early information about a baby's health. Typically, first trimester screening is done between weeks 11 and 14 of pregnancy — earlier than many other prenatal tests.
First trimester screening includes two steps:
- A blood test to measure levels of two pregnancy-specific substances
- An ultrasound exam to measure the size of the clear space in the tissue at the back of a baby's neck (nuchal translucency)
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 (trisomy 21) or Edwards syndrome (trisomy 18). Down syndrome causes mental retardation. Edwards syndrome causes more severe retardation and is often fatal by age 1.
If your risk level is low, first trimester screening can offer reassurance of a healthy pregnancy. If your risk level is high enough to cause concern, you may choose to follow first trimester screening with diagnostic testing — such as chorionic villus sampling or amniocentesis. These tests are more invasive but can reliably determine whether a baby actually has Down syndrome or Edwards syndrome.
- Canick JA, et al. First trimester and integrated screening for Down syndrome and trisomy 18. http://www.uptodate.com/home/index.html. Accessed March 8, 2010.
- Benacerraf BR. Sonographic findings associated with aneuploidy. http://www.uptodate.com/home/index.html. Accessed March 8, 2010.
- Chervenak FA, et al. Ethical considerations in first-trimester Down syndrome risk assessment. Current Opinion in Obstetrics and Gynecology. In press. Accessed March 8, 2010.
- Kagan KO, et al. Screening for trisomy 21 by maternal age, fetal nuchal translucency thickness, free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A. Ultrasound in Obstetrics & Gynecology. 2008;31:618.
- Screening for birth defects. American Congress of Obstetricians and Gynecologists. http://www.acog.org/publications/patient_education/bp165.cfm. Accessed March 8, 2010.
- Routine tests in pregnancy. American Congress of Obstetricians and Gynecologists. http://www.acog.org/publications/patient_education/bp133.cfm. Accessed March 8, 2010.
- Harms RW (expert opinion). Mayo Clinic, Rochester, Minn. April 11, 2010.
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