The effect of iron treatment on fetal fraction according to gestational weeks in maternal anemia

Objective The aim of this study was to evaluate the effects of iron (Fe) supplementation in maternal anemia on fetal fraction (FF) according to different gestational ages (GA) in pregnancies undergoing non-invasive prenatal testing (NIPT). FF is one of the key parameters that directly determines the analytical reliability of NIPT. Therefore, understanding the impact of hematologic and treatment-related variables on FF is important for optimal test timing and for minimizing false-negative results. Methods This retrospective, single-center study included 308 pregnant women who underwent NIPT between January 2020 and December 2025. Cases with obesity, multiple pregnancies, in vitro fertilization conception, chronic systemic diseases, or medication use were excluded to obtain a homogeneous cohort. Hematologic parameters and FF values were evaluated according to GA and Fe supplementation status. Correlation, subgroup, and multivariable regression analyses were performed using non-parametric statistical methods. Results The median maternal age was 33.5 years, and the median GA was 14.43 weeks. A moderate positive correlation was observed between GA and FF ( ρ = 0.399, p 0.001), whereas Hb levels showed only a weak positive correlation with FF ( ρ = 0.112, p = 0.050). No significant associations were found between FF and hematocrit, white blood cell, or platelet counts. The median FF was significantly higher after 16 weeks of gestation (11.7% vs. 8.3%, p 0.001). Notably, among pregnancies ≥16 weeks, FF was significantly lower in women receiving Fe supplementation compared with those who did not ( p = 0.010), whereas no significant difference was observed before 16 weeks. In multivariable regression analysis adjusting for GA, Hb level at the time of NIPT, and MA, only GA remained independently associated with FF. Conclusion Although crude analyses suggested lower FF values among women receiving Fe supplementation after 16 weeks of gestation, multivariable regression analysis indicated that GA remained the primary determinant of FF. Therefore, the apparent association between Fe supplementation and FF likely reflects underlying GA-related and hematologic differences rather than a direct effect of Fe therapy.