Background: Previous studies investigated the association between gestational anemia and neonatal outcomes. However, few studies explored whether the effects of gestational anemia could be eliminated by subsequent correction of anemia in the later stages of pregnancy. This study aimed to investigate the relationship between anemia in different trimesters and neonatal outcomes.

Methods: The study was conducted in Shanghai, China, with a sample of 46,578 pregnant women who delivered between January 1, 2016 and July 1, 2019. A multivariable logistic regression model was adopted to analyse the associations between maternal anemia and neonatal outcomes.

Results: The incidence of gestational anemia was 30.2%, including 4.4% in the first trimester, 9.6% in the second trimester, and 16.2% in the third trimester. Only 24.5% (507/2066) of anemia that occurred in the first trimester and 29.6% (1320/4457) that occurred in the second trimester could be corrected in the later stages of pregnancy. Anemia occurring in the first trimester was associated with small for gestational age [odds ratio (OR) 1.46; 95% confidence interval (CI) 1.20-1.78] and with fetal distress (OR 1.23; 95% CI 1.08-1.40). Anemia corrected in the first trimester also was associated with a higher risk of small for gestational age.