Sex-Specific Variation in Maternal Serum Screening Markers Across the First and Second Trimesters: Evidence from 10,384 Screened Pregnancies

<b>Background</b>: Maternal serum screening remains widely implemented for prenatal aneuploidy assessment despite increased uptake of cell-free DNA testing. Evidence suggests that fetal sex may influence placental endocrine function and maternal serum biomarker levels; however, available studies are inconsistent and often limited by sample size or incomplete adjustment for maternal factors. <b>Objective</b>: The aim of this study was to determine whether fetal sex independently modifies first- and second-trimester maternal serum marker Multiple of the Median (MoM) values and whether sex-related biochemical variation affects trisomy-21 screen-positive classification. <b>Methods</b>: A retrospective cohort was identified from institutional screening records (10,384 screened pregnancies), of which 5040 first-trimester and 1476 second-trimester cases had complete biochemical data. First-trimester PAPP-A and free β-hCG, as well as second-trimester AFP, uE3, and free β-hCG, were measured. Implausible MoM values (<0.10 or >5.00) were excluded. Multivariable linear and logistic regression models adjusted for maternal age, maternal weight, gestational age at sampling, and parity assessed independent associations. <b>Results</b>: Pregnancies with female fetuses showed significantly higher MoM values for first-trimester PAPP-A and free β-hCG as well as second-trimester AFP and uE3. The magnitude of these differences was small, and no significant differences were observed in trisomy-21 screen-positive rates between fetal sex groups. <b>Conclusions</b>: Fetal sex independently influences several maternal serum markers across both trimesters but does not result in clinically meaningful differences in trisomy-21 screening outcomes under current algorithms. Any potential relevance of fetal sex for risk interpretation should be regarded as hypothesis-generating and requires outcome-validated investigation before clinical application.