Folate Receptor Alpha Autoantibodies in Early Pregnancy: First-Trimester Reference Intervals and Proposed Clinical Thresholds.

Maternal folate receptor alpha autoantibodies (FRAA) have been associated with impaired placental folate transport, fetal cerebral folate deficiency (CFD) and neurodevelopmental risks including autism spectrum disorder (ASD); however, first-trimester-specific reference intervals remain undefined. This prospective single-center study of 534 healthy pregnant women at 10 + 0 to 15 + 6 weeks' gestation (week 10 n = 26; week 11 n = 155; week 12 n = 203; week 13 n = 105; week 14 n = 39; week 15 n = 6) used a CE-IVDR FRAA ELISA, following CLSI EP28-A3c and IFCC C-RIDL protocols, to establish week-specific percentiles (P5, P50, P95, P99) via non-parametric estimation and log-smoothed regression with a 1-week rolling window. An internal-consistency ROC analysis was performed against the within-dataset ≥P99 designation and is therefore not interpretable as discrimination against an independent clinical outcome. Median FRAA declined from 29 ng/mL (week 10) to 25 ng/mL (week 14), with provisional clinically actionable thresholds of P95 ≈ 120 ng/mL and P99 ≈ 150 ng/mL. These data provide the first first-trimester normative percentile curves for maternal FRAA and may support prioritizing FRAA assessment before 15 weeks (onset of accelerated transplacental IgG transfer). Given the cross-sectional, single-center design, the small samples at weeks 14-15, and the absence of long-term neurodevelopmental outcome data, the proposed thresholds and any downstream clinical implications, including folinic acid intervention and ASD risk mitigation, should be considered hypothesis-generating and require external and longitudinal validation.