Screening Tests for Pregnant Women (identifying pregnancies with high risk of Down Syndrome etc.)

First trimester screening
(Nuchal Translucency, PAPP-A and Free b-hCG)

Screening test for identifying a risk for Down syndrome in the fetus in the first trimester of the pregnancy.

This test combines an ultrasonic measuring of the thickness of the nuchal fold translucency of the fetus, and a biochemical test for PAPP-A and Free b-hCG in maternal blood. In calculating the risk, the age of the woman is also brought into account. In case the nuchal translucency exceeds 3.0 mm or other defects are identified, the woman will be immediately referred to genetic counseling, and diagnostic tests (amniocenthesis, CVS) will be performed according to need.

If the nuchal translucency is less than 3.0 mm and no defects were identified in the fetus, the woman will wait for the results of the blood test and for the calculation of the weighed risk. The answers are obtained within approximately a week. In case there is high risk for Down syndrome, the woman will be referred to genetic counseling for the option of an invasive test (amniocenthesis or CVS).

Although the test is relatively new, it is already in routine use in several countries. The United Kingdom Royal College of Obstetricians and Gynaecologists (RCOG) states that “there are sufficient data to consider screening for Down syndrome by the measurement of nuchal translucency at 10–14 weeks' gestation an acceptable procedure”, and that “there is now sufficient evidence to consider that specific serum markers for Down syndrome at 9-13 weeks gestation (notably PAPP-A and free BhCG may be as effective as those serum markers in established use at 15-22 weeks gestation”.

In scientific conferences it is emphasized that the first trimester screening must rely on a combination between the nuchal translucency test and the biochemical tests, because no test in itself obtains a high enough rate of risk identification and a low enough false positive rate.

Calculating the risk

A “first trimester screening” may identify about 80% (perhaps even more) of the fetuses with Down syndrome and other chromosomal aberrations. The risk is calculated by a unique software developed by one of the leading researchers in the world in this field, Prof. Nicholas Wald.

The software has been adapted to suit the local Israeli population, based on hundreds of laboratory and ultrasound tests performed in the medical center in Tel Aviv.

The early identification of the group of women at higher risk, enables an early diagnostic test by Chorionic Villus Sampling. In case a chromosomal disorder is found, an early abortion can be carried out using suction curettage, a procedure involving less medical complications and psychological difficulty than the termination at the second trimester.