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 extra sequences of chromosome 18 (trisomy 18).
First trimester screening, also called the first trimester combined test, 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 (HCG)
- 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.
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 with Down syndrome or trisomy 18.
If results show that your risk level is moderate or high, you might choose to follow first trimester screening with another test that's more definitive.
Oct. 09, 2015
- Messerlian GM, et al. First trimester combined and integrated tests for screening for Down syndrome and trisomy 18. http://www.uptodate.com/home. Accessed Aug. 12, 2015.
- Benacerraf BR. Sonographic findings associated with fetal aneuploidy. http://www.uptodate.com/home. Accessed Aug. 12, 2015.
- Messerlian GM, et al. Laboratory issues related to maternal serum screening for Down syndrome. http://www.uptodate.com/home. Accessed Aug. 6, 2015.
- Frequently asked questions. Diagnostic tests for birth defects FAQ164. American College of Obstetricians and Gynecologists. http://www.acog.org/Patients/FAQs/Diagnostic-Tests-for-Birth-Defects. Accessed Aug. 12, 2015.
- Messerlian GM, et al. Down syndrome: Overview of prenatal screening. http://www.uptodate.com/home. Accessed Aug. 6, 2015.
- Ostermaier KK. Management of Down syndrome. http://www.uptodate.com/home. Accessed Aug. 12, 2015.
- Cunningham FG, et al. Prenatal diagnosis. In: Williams Obstetrics. 24th ed. New York, N.Y.: The McGraw-Hill Companies; 2014. http://www.accessmedicine.com. Accessed Aug. 3, 2015.
- Fischbach FT, et al. Prenatal diagnosis and tests of fetal well-being. In: A Manual of Laboratory and Diagnostic Tests. 9th ed. Philadelphia, Pa.: Lippincott Williams & Wilkins; 2015.
- Beckman CRB, et al. Genetics and genetic disorders in obstetrics and gynecology. In: Obstetrics and Gynecology. 7th ed. Baltimore, Md.: Lippincott Williams & Wilkins; 2014.
- AskMayoExpert. Prenatal testing and diagnostic screening. Rochester, Minn.: Mayo Foundation for Medical Education and Research; 2014.
- Wolfberg A, et al. Noninvasive prenatal testing using cell-free nucleic acids in maternal blood. http://www.uptodate.com/home. Accessed Aug. 7, 2015.
- American College of Obstetricians and Gynecologists (ACOG) Committee on Practice Bulletins — Obstetrics. ACOG Practice Bulletin No. 77: Screening for fetal chromosomal abnormalities. Obstetrics & Gynecology. 2007;109:217. Reaffirmed 2013.
- American College of Obstetricians and Gynecologists Committee on Genetics. Committee Opinion No. 640: Cell-free screening for fetal aneuploidy. Obstetrics & Gynecology. 2015;126:e31.