What Does the First Trimester Screening Test For?

Reviewed on 5/25/2021

Between the 11th and 13th week of pregnancy, a woman will have an ultrasound and blood screening test to estimate the probability a genetic disorder and identify risks for specific chromosomal abnormalities, including Down Syndrome (trisomy-21), trisomy-18, and trisomy 13. The first trimester screening test is not a diagnostic test and does not provide a definitive diagnosis.
Between the 11th and 13th week of pregnancy, a woman will have an ultrasound and blood screening test to estimate the probability a genetic disorder and identify risks for specific chromosomal abnormalities, including Down Syndrome (trisomy-21), trisomy-18, and trisomy 13. The first trimester screening test is not a diagnostic test and does not provide a definitive diagnosis.

The first trimester of pregnancy refers to the first three months of pregnancy. The baby grows fastest during the first trimester. By the end of the first trimester, the baby’s bones, muscles, and organs have formed and it is now considered a fetus. The fetus can weigh about 0.5 to 1 ounce and measure an average of 3 to 4 inches in length.

When Is the First Trimester Screening Test?

  • A first trimester screening test is an optional test performed between the 11th and 13th week of pregnancy that combines a maternal blood screening test with an ultrasound evaluation of the fetus to identify risks for specific chromosomal abnormalities, including Down Syndrome (trisomy-21), trisomy-18, and trisomy 13.
  • First trimester screening tests can detect about 84% of babies with Down syndrome, and about 80% of babies with trisomy 18.
  • Another portion of the test (the nuchal translucency) can help identify other fetal abnormalities, such as heart problems.

What Conditions Are Found During the First Trimester Screening?

  • The first trimester screening test is not a diagnostic test, which means it does not definitively diagnose the baby with Down syndrome, trisomy 13, or trisomy 18.
  • The purpose of the screening test is to estimate the probability a genetic disorder is present. It is based on the mother’s age, information from the ultrasound, and blood testing. It is possible to have an abnormal test and later find the estimate is false.
  • The testing is an informational tool to let a woman and her doctor know that further diagnostic tests may be considered. 

What Happens After the Genetic and Chromosomal Testing?

  • If the screening results indicate a baby is at increased risk for Down syndrome or trisomy 13 or 18, parents may be referred to a genetic counselor to review the results and to discuss additional diagnostic testing options such as chorionic villus sampling (CVS) and amniocentesis.
  • CVS and amniocentesis are more than 99% accurate in diagnosing chromosomal abnormalities. 
  • The first trimester screening test does not provide information about other potential chromosome conditions, other genetic syndromes, genetic disorders, birth defects, or causes of mental retardation.

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Reviewed on 5/25/2021
References
https://www.hopkinsmedicine.org/health/wellness-and-prevention/the-first-trimester

https://americanpregnancy.org/prenatal-testing/first-trimester-screening-730/

https://www.hopkinsmedicine.org/gynecology_obstetrics/specialty_areas/maternal_fetal_medicine/services/fetal_diagnostic_procedures/down_syndrome_screening_information.html