Key Points
In pregnant women, physiological ferritin thresholds for iron deficiency are <25 µg/L in the 1st and <20 µg/L in 2nd and 3rd trimesters.
These physiologically identified ferritin thresholds for iron deficiency track concentrations of hepcidin, the iron regulatory hormone
Serum ferritin (SF) concentration is the most widely used indicator for iron deficiency (ID). During pregnancy, the World Health Organization recently recommended SF thresholds for ID of <15 µg/L for the 1st trimester of pregnancy, based on expert opinion, and made no recommendations for the 2nd and 3rd trimesters. We examined the relationship of SF with two independent indicators of the onset of iron-deficient erythropoiesis, hemoglobin (Hb) and soluble transferrin receptor 1 (sTfR1), in cross-sectional data from NHANES for 1999-2010 and 2015-2018. We included 1288 pregnant women 15-49 years and excluded women with inflammation or potential liver disease. We used restricted cubic spline (RCS) regression analysis to determine SF thresholds for iron-deficient erythropoiesis. SF decreased during pregnancy; geometric mean SF was higher during the 1st and lower during the 2nd and 3rd trimesters. Using RCS analysis, the SF thresholds identified during pregnancy were <25.8 (18.1, 28.5) µg/L during 1st trimester, <18.3 (16.3, 22.9) µg/L during 2nd trimester, and <19.0 (14.4, 26.1) µg/L during 3rd trimester. These SF threshold levels track concentrations of hepcidin, the iron regulatory hormone controlling the mobilization of iron stores. A SF concentration of <15 µg/L as the criterion for ID may underestimate the true prevalence of ID throughout pregnancy. In our study, an additional one of every ten pregnant women would be recognized as iron deficient by using the physiologically based thresholds at SF of about 25 µg/L during the 1st and of about 20 µg/L during the 2nd and 3rd trimesters.
