First Trimester Screening (PAPP-A and hCG) Test

The first trimester screening is a combination of test for PAPP-A, HCG and nuchal translucency which are usually used to assess the risk which the fetus a pregnant woman is carrying has a chromosome disorder such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). The patients may be first offered for the first trimester screen for prenatal Down syndrome screening and other options offered includes the second trimester maternal serum screen and the cell-free fetal DNA test.

The heath care practitioner may offer the test between 11 and 14 weeks of pregnancy. Patients are not required to do anything special while preparing for first trimester screening and can eat or drink normally before both the blood test and the ultrasound exam.

A mathematical calculation using the obtained results from the PAPP-A, HCG is used to determine a numeric risk of a chromosomal defect in the fetus and the risk is usually compared with an established cutoff. If the risk is higher than the cut off value (e.g., probability of 1 in 230 or higher), then it is considered a positive screen and the woman may be at an increased for having a baby with a chromosomal abnormality. Always remember that the positive screening tests are not diagnostic of fetal abnormalities while they do indicate an increased risk and only a very small number of women with a positive first trimester screen have babies who actually have a chromosome disorder. If a screening test comes out to be positive, more definitive tests are needed to determine and confirm a diagnosis. Screening will not detect all cases of fetal abnormalities.

Test results are very dependent on nuchal translucency techniques and the accurate determination of the gestational age of the fetus. The gestational age of the fetus has to be accurately determined, if not, the results may be either falsely high or low. The nuchal translucency ultrasound is an assessment which is somehow unique to each fetus and can be done independently with ultrasound. It’s very important to remember that a woman with a multiple gestation pregnancy, the patient should consult her health care practitioner about her options.