Objectives: Early screening for Down syndrome is desirable so that more time is left for intervention in the event of a positive test. In retrospective first trimester studies, maternal serum free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A have been reported as useful markers. Our objective was to confirm these results in a prospective study carried on an unselected population.
Design and methods: In a cohort of pregnant women recruited prospectively between 9 and 13 weeks' gestation, we measured maternal free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A in all affected pregnancies and 500 representative uneffected pregnancies. Serum concentrations were transformed to multiples of the median value in normal pregnancies of the same length of gestation, and rates of detection of various combinations of the markers were estimated by multivariate analysis.
Results: Down syndrome was observed in 18 fetuses from the 10, 160 women recruited. Levels of free beta-human chorionic gonadotropin were elevated in affected pregnancies with an overall median value 1.8 times the median of women with normal pregnancies while pregnancy-associated plasma protein A was significantly diminished (0.51 multiples of the median). At a fixed false-positive risk of 10%, 33% (11-55), 50% (27-73), 44% (11-67), and 67% (45-89) of the affected pregnancies would have been detected (95% CI) with maternal age alone or combined with with free beta-human chorionic gonadotropin, pregnancy-associated plasma protein A or both, respectively.
Conclusions: We confirm in a prospective noninterventional study that maternal serum free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A can be used in the first trimester of pregnancy to screen for Down syndrome with a performance similar to second trimester screening programs.