The first-trimester prenatal screening is a combination of tests for PAPP-A, free beta hCG, and nuchal translucency that are used to assess the risk that the fetus a pregnant woman is carrying has a chromosome disorder such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18).
Pregnancy-associated plasma protein A (PAPP-A) is a protein produced first by the growing placenta. During a normal pregnancy, levels of this protein increase in the pregnant woman’s blood until delivery.
Human chorionic gonadotropin (hCG) is a hormone produced during pregnancy in large quantities by the placenta. Either free beta subunit or total hCG can be used in first-trimester screening. Levels of both usually rise rapidly in the pregnant woman’s blood for the first 8 to 10 weeks, then decrease and stabilize at a lower level for the remainder of the pregnancy.
Nuchal translucency is a measurement made by ultrasound. The ultrasonographer measures the fluid collection between the spine and the skin at the nape of the fetus’s neck. It is a procedure that requires a specially trained radiologist, proper alignment of the fetus, and careful measurement. It is not a routine ultrasound.
If the results of the first-trimester screening are cause for concern, diagnostic tests such as amniocentesis or chorionic villus sampling (CVS) may be recommended.
One advantage of the first-trimester screening is that it gives expecting women the option of earlier screening. However, the first-trimester screen does not assess the risk of neural tube defects such as spina bifida. This may be assessed in the second trimester as part of the maternal serum screen (triple or quad screening tests).
The first-trimester prenatal screening test is one of the options for prenatal Down syndrome screening. Other options include the second-trimester screen tests (alpha and beta tests) and non-invasive prenatal screening.