Performance Evaluation of Biochemical and Ultrasound Markers for the Screening of Chromosomal Numerical Abnormalities in the First Trimester of Pregnancy

Background:To estimate the effectiveness of the first-trimester combined screening test in our population, departing from the results of diagnostic sensitivity and false positive rate (FPR), and checking some important parameters in prenatal screening. Methods: The test was evaluated on 14250 pregnant women. The following variables were studied: the number of invasive techniques and the reasons for using such techniques, newborns with chromosomal abnormalities, total number of births, variation of biochemical markers throughout the gestational weeks, and MoM (multiple of the median) for biochemical and ultrasound markers. Results: An important coverage and a decreased number of invasive techniques were obtained. For our population of pregnant women, the best gestational week to determine free beta-hCG and PAPP-A would be week 11 in which the best discrimination was found between affected and non affected fetuses for the three trisomies researched. We propose the cut-off 1/350, because it is the best one to increase sensitivity without exceeding the 5% FPR. Conclusions: Combined screening should be offered to pregnant woman, preferentially at week 11. Although different cut-offs for this prenatal test have been recommended by scientific societies, biochemical laboratories should set their own cut-off for getting the best sensitivity and FPR results. There should be a good level of collaboration between the laboratory and each participating ultrasound unit in order to ensure an optimal use of the first-trimester combined screening test.