Pregnancy Outcomes Associated With Anemia in the First Trimester and Anemia Resolution by Late Pregnancy

OBJECTIVE: Evidence related to anemia in early pregnancy, and its resolution or persistence by late pregnancy, is limited. We evaluated pregnancy outcomes associated with anemia in early pregnancy and resolution compared with persistence in late pregnancy. METHODS: We used the Merative™ Marketscan ® Commercial Database of nationwide insurance claims (2018-2023) and included pregnant individuals without hereditary anemias. We used hemoglobin and hematocrit to identify anemia in early pregnancy (before 14 weeks of gestation) and late pregnancy (at or after 24 weeks of gestation). Pregnancy outcomes included preeclampsia, placenta previa, placental abruption, severe postpartum hemorrhage, blood products transfusion, cesarean birth, nontransfusion severe maternal morbidity (SMM), spontaneous preterm birth, medically indicated preterm birth, and small-for-gestational-age (SGA) birth weight. We used modified Poisson regression to estimate associations between: 1) anemia in early pregnancy and pregnancy outcomes; and 2) anemia resolution by late pregnancy and pregnancy outcomes, adjusting for confounders by inverse probability weighting. RESULTS: Among 73,586 individuals, 4.4% (95% CI, 4.3-4.6%) had anemia in early pregnancy. Early pregnancy anemia was associated with higher risk of each outcome assessed, with the exception of placenta previa, with the highest associated risk of blood products transfusion (2.4% vs 0.8%; adjusted risk ratio [aRR] 2.45; 95% CI, 1.91-3.13). Of those with early pregnancy anemia and laboratory values in late pregnancy (72.1%), 53.4% had persistent anemia and 46.6% had resolved anemia. Persistent anemia was associated with nontransfusion SMM (2.6% vs 1.1%, aRR 1.64; 95% CI, 1.13-2.37), blood products transfusion (2.9% vs 0.8%, aRR 2.60; 95% CI, 1.84-3.69), and SGA birth weight (8.5% vs 6.8%, aRR 1.23; 95% CI, 1.01-1.50), compared with those without anemia in the first trimester. The resolution of anemia by late pregnancy was not associated with nontransfusion SMM (1.6% vs 1.1%; aRR 1.07; 95% CI, 0.65-1.74) but was associated with blood products transfusion (1.6% vs 0.8%; aRR 1.64; 95% CI, 1.01-2.67) and SGA birth weight (10.0% vs 6.8%; aRR 1.38; 95% CI, 1.15-1.67) compared with those without anemia in the first trimester. CONCLUSION: Anemia in the first trimester was associated with adverse maternal and neonatal outcomes. The resolution of anemia by late pregnancy eliminated the association with nontransfusion SMM but not other outcomes, emphasizing the importance of treating anemia in early pregnancy and before pregnancy.