The impact of temporal variability of biochemical markers PAPP-A and free β-hCG on the specificity of the first-trimester Down syndrome screening: a Croatian retrospective study

There were no significant differences between the sub-groups, regarding maternal age, maternal weight and the proportion of smokers. The difference in log10 MoM free β-hCG values, between the 11th and 12th gestational week, was significant (p = 0.002). The difference in log10 MoM PAPP-A values between the 11th and 12th, and between 12th and 13th week of gestation was significant (p = 0.006 and p = 0.003, respectively). False-positive rates of biochemical risk for trisomies were 16.1% before the 11th week, 12.8% in week 12th, 11.9% in week 13th and 9.9% after week 13th. The differences were not statistically significant.Biochemical markers (log10 MoMs) showed gestation related variations in the first-trimester unaffected pregnancies, although the variations could not be attributed either to the inaccuracy of analytical procedures or to the inappropriately settled curves of median values for the first-trimester biochemical markers.The first-trimester screening for trisomies 21, 18 and 13, combining maternal age, fetal nuchal translucency thickness (NT), maternal serum free β-human chorionic gonadotropin (free β-hCG) and pregnancy associated plasma protein-A (PAPP-A) has revealed a highly potential possibility of an early detection of fetal aneuploidies [1-5].Various co-variables influence the specificity and sensitivity of the first-trimester biochemical markers have been studied [6]. Corrections of the calculated gestational MoM values (Multiples of the Median) have been proposed for maternal weight, cigarette consumption, ethnicity and twin-pregnancy [7-10]. Fetal gender, maternal insulin-dependent diabetes mellitus, parity/gravidity and procedures in assisted reproduction show certain, but insignificant influence on the screening performance [11-15].Several studies have focused on the optimal timing for analysis of biochemical markers, predominantly concerning their sensitivity for fetal trisomies [16,17]. The optimal sensitivity of PAPP-A is between 9-12 weeks' ge