Hemoglobin During Pregnancy Does Not Mediate the Relationship between Nutrition Supplements and Intrauterine Growth: A Secondary Data Analysis of Women First Preconception Nutrition Trial

Background: Nutrition supplements such as multiple micronutrient-fortified small-quantity lipid-based nutrient supplementation (SQ-LNS) consumed either before or during pregnancy have been shown to improve intrauterine growth, but the mechanisms through which the supplements improve intrauterine growth remain unclear. Objectives: We examined whether hemoglobin (Hb) during pregnancy could be a potential mechanism through which multiple micronutrient-fortified SQ-LNS improve intrauterine growth. Methods: We used data collected from women and newborns in a randomized controlled trial conducted in Pakistan, India, the Democratic Republic of the Congo, and Guatemala. Women were randomly assigned to consume multiple micronutrient-fortified SQ-LNS from preconception until birth (arm 1); consume the SQ-LNS from the second trimester of pregnancy until birth (arm 2); or no supplement (arm 3). Intrauterine growth, expressed as birth length, weight, and head circumference Z-scores, was the outcome. The mediator was Hb (g/dL) measured at 12 (n = 2075) and 32 wk of gestation (n = 2157). Causal mediation analysis was employed to estimate direct and indirect effects. Results: Hb levels at 12 or 32 wk of gestation did not mediate the relation between the SQ-LNS and intrauterine growth. Indirect effects of preconception SQ-LNS (arm 1) compared with arm 3, mediated by Hb at 12 wk of gestation, were 0.02 [95% confidence interval (CI): –0.02, 0.01], 0.01 (95% CI: –0.01, 0.02), and 0.01 (95% CI: –0.01, 0.02) for length, weight, and head circumference Z-scores, respectively. The corresponding direct effects (95% CIs), not mediated by Hb, were 0.18 (0.09, 0.33), 0.12 (0.03, 0.23), and 0.06 (–0.03, 0.20), respectively. Site-specific and gestational age-adjusted data analyses at 12 and 32 wk of gestation confirmed the findings of no statistically significant mediated effects of Hb during pregnancy. Conclusions: The observed main effect of multiple micronutrient-fortified SQ-LNS on intrauterine growth was not mediated by Hb levels at 12 or 32 wk of gestation. The findings suggest exploring other pathways implicated in the association between the SQ-LNS and intrauterine growth. This trial was registered at clinicaltrials.gov as #NCT01883193 (https://clinicaltrials.gov/ct2/show/NCT01883193?term=01883193&rank=1).