The combination of tests for PAPP-A, hCG and nuchal translucency that are included in the first trimester screen are used to assess the risk that the fetus a pregnant woman is carrying has a abnormality such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18).
The first trimester screen has not been as widely used as the quad screen that is offered in the second trimester, but it has begun to gain acceptance as the medical community has pursued ways to screen for Down syndrome earlier in pregnancy. The first trimester screen does not, however, assess risk of neural tube defects, such as , which can be done with the quad screen.
There are several approaches to screening depending on what technology is available and when the woman first comes in for prenatal care. For more information, see Common Questions #4.
A mathematical calculation using the results obtained from the PAPP-A, hCG, and nuchal translucency ultrasound is used to determine a numeric risk of a defect in the fetus. This risk is compared with an established cut-off. If the risk is higher than the cut-off value (e.g., probability of 1 in 300 or higher), then it is considered a positive screen and the woman may be at increased risk for having a baby with a chromosomal abnormality.
In pregnancies where the fetus is carrying a chromosomal defect, such as the extra chromosome material that results inDown syndrome or Edwards syndrome, the levels of PAPP-A tend to be decreased, the levels of hCG are significantly increased, and the space at the fetus's neck is larger than normal.
The interpretation of these results should be provided by a or clinician who can explain the meaning of the results and offer choices about follow up. It is important to remember that screening tests are not diagnostic of fetal abnormalities but indicate a normal or increased risk.
While the first trimester screen can correctly identify approximately 85% of women carrying a fetus with Down syndrome and up to 75% of those with Edwards syndrome, about 5% of normal pregnancies will have a result.
If a screening test is positive, more definitive tests are needed to determine and confirm a diagnosis. These may include a diagnostic test such as chorionic villus sampling (CVS) in the first trimester or amniocentesis in the second trimester. While these two procedures are more accurate than first or second trimester screening, they are also invasive and carry a small risk of miscarriage and a rare risk of injury to the fetus.
Screening will not detect all cases of fetal abnormalities.
Test results are very dependent on nuchal translucency techniques and the accurate determination of the age of the fetus. If the gestational age of the fetus has not been accurately determined, the results may be either falsely high or low.
In multiple gestation pregnancies (twins, triplets, etc.), calculation of the risk of Down syndrome or Edwards syndrome can be difficult because the amount of PAPP-A and free beta hCG is increased. The nuchal translucency ultrasound, however, is an assessment that is unique to each fetus and can be done independently with ultrasound. Women with a multiple gestation pregnancy should consult their doctor about their options.