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Background  The variability of maternal serum biochemical markers for Down syndrome, free β-hCG and PAPP-A can have a different impact on false-positive rates between the 10+0 and 13+6 week of gestation. The study population comprised 2883 unaffected, singleton, spontaneously conceived pregnancies in Croatian women, who delivered apparently healthy child at term. Women were separated in 4 groups, dependently on the gestational week when the analyses of biochemical markers were performed. The concentrations of free β-hCG and PAPP-A in maternal serum were determined by solid-phase, enzyme-labeled chemiluminiscent immunometric assay (Siemens Immulite). Concentrations were converted to MoMs, according to centre-specific weighted regression median curves for both markers in unaffected pregnancies. The individual risks for trisomies 21, 18 and 13 were computed by Prisca 4.0 software.    Findings  There were no significant differences between the sub-groups, regarding maternal age, maternal weight and the proportion of smokers. The difference in log 10  MoM free β-hCG values, between the 11 th  and 12 th  gestational week, was significant (p = 0.002). The difference in log 10  MoM PAPP-A values between the 11 th  and 12 th , and between 12 th  and 13 th  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 11 th  week, 12.8% in week 12 th , 11.9% in week 13 th  and 9.9% after week 13 th . The differences were not statistically significant.    Conclusions  Biochemical markers (log 10  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.

bmc research notes

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