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First-Trimester Screening


First trimester screening is a prenatal test that offers early information about a baby’s health. First trimester screening is performed between weeks 11 and 14 of pregnancy, which is earlier than many other prenatal tests. This screening process can help to determine the risk of the fetus having certain birth defects such as Down syndrome, other more serious chromosomal abnormalities, heart defects and other genetic conditions.   

First trimester screening is a combination of a blood test to measure levels of three pregnancy-specific substances-- pregnancy-associated plasma protein-A (PAPP-A), alpha feto protein, and human chorionic gonadotropin (hCG) --- and a specialized ultrasound where a nuchal translucency (NT) measurement is obtained and the presence of absence of the nasal bone is documented.  The NT is a specific area at the back of your baby's neck that typically has a small amount of fluid.  Babies with abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this space to be larger than average.  Also, babies who do not have a nasal bone have a greater risk for chromosomal and genetic abnormalities.

You don't need to do anything special to prepare for first trimester screening. You can eat and drink normally before both the blood test and the ultrasound exam.

Using your age and the results of the blood test and the ultrasound, your health care provider can modify your risk of carrying a baby who has Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13).  Down syndrome causes varying degrees of mental retardation with some physical anomalies possible. Edwards and Patau syndromes cause more severe retardation and physical anomalies that may be life threatening.

ACOG (American College of Obstetrics and Gynecology) states “first-trimester screening using both nuchal translucency measurement and biochemical markers is an effective screening test for Down syndrome in the general population. …this screening strategy results in a higher Down syndrome detection rate than does the second-trimester maternal serum triple screen…”. They also state “screening and invasive diagnostic testing for aneuploidy should be available to all women who present for prenatal care before 20 weeks of gestation regardless of maternal age.”

Like other screening tests, a first trimester screening won't give you a diagnosis but if your risk level is low, first trimester screening can offer reassurance. If your risk level is moderate or high, you may choose to follow first trimester screening with non-invasive prenatal testing (NIPT) or a more invasive, diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis to determine whether the baby actually has a chromosome abnormality.