Fetal iron uptake from recent maternal diet and the maternal RBC iron pool

Background During pregnancy iron can be obtained from the diet, body iron stores, or iron released from RBC catabolism. Little is known about the relative use of these sources to support fetal iron acquisition. Objectives To describe longitudinal change in iron absorption and enrichment across gestation and partitioning of RBC iron to the fetus. Methods Fifteen pregnant women ingested an oral stable iron isotope (Fe-57) in the second trimester (T2) of pregnancy (weeks 14-16) to label the RBC pool, and a second oral stable isotope (Fe-58) in the third trimester (T3) (weeks 32-35). Absorption was measured at T2 and T3. Change in RBC Fe-57 enrichment was monitored (18.8-26.6 wk) to quantify net iron loss from this pool. Iron transfer to the fetus was determined based on RBC Fe-57 and Fe-58 enrichment in umbilical cord blood at delivery. Results Iron absorption averaged 9% at T2 and increased significantly to 20% (P = 0.01) by T3. The net increase in iron absorption from T2 to T3 was strongly associated with net loss in maternal total body iron (TBI) from T2 to T3 (P = 0.01). Mean time for the labeled RBC Fe-57 turnover based on change in RBC enrichment was 94.9 d (95% CI: 43.5, 207.1 d), and a greater decrease in RBC Fe-57 enrichment was associated with higher iron absorption in T2 (P = 0.001). Women with a greater decrease in RBC Fe-57 enrichment transferred more RBC-derived iron to their fetus (P < 0.05). Conclusions Iron absorption doubled from T2 to T3 as maternal TBI declined. Women with low TBI had a greater decrease in RBC iron enrichment and transferred more RBC-derived iron to their neonate. These findings suggest maternal RBC iron serves as a significant source of iron for the fetus, particularly in women with depleted body iron stores.