Relationship between pre-pregnancy body mass index and postpartum depression, and the mediation effects of cesarean delivery, low birth weight, child anomaly, and breastfeeding: a prospective birth cohort in the Japan environment and children’s study

BACKGROUND: Whereas underweight and obesity have been reported as risk factors for postpartum depression (PD), in Japan, younger women tend to express a stronger preference to have a lower body mass index (BMI). In this study, we aimed to examine the association between pre-pregnancy BMI categories and PD among Japanese pregnant women, and conducted an exploratory analysis of the mediating roles of adverse birth outcomes. METHODS: (obesity). PD was assessed using the Japanese version of the Edinburgh Postnatal Depression Scale at 1 and 6 months after childbirth (1mo and 6mo, respectively). Potential mediators included adverse birth outcomes, including cesarean delivery, low birth weight, child anomalies, and non-initiation of breastfeeding by 1mo. Logistic regression models estimated adjusted odds ratios, and a causal mediation analysis was conducted to decompose the total effect of pre-pregnancy BMI into natural direct and indirect effects through the candidate mediators. RESULTS: Severe-moderate underweight, overweight, and obesity were significantly associated with PD at 1mo and 6mo (1mo: OR = 1.21, 95% confidence interval [CI]: 1.08‒1.35; 6mo: OR = 1.16, 95% CI: 1.03‒1.31), and upper-normal BMI was significantly associated with PD at 1mo (OR = 1.09, 95% CI: 1.02‒1.17). Significant mediating effects of low birth weight were observed among lower BMI categories, child anomalies among higher BMI categories, and non-initiation of breastfeeding among both BMI categories. CONCLUSIONS: Pre-pregnancy severe‒moderate underweight, overweight, and obesity were significant risk factors for PD at 1mo and 6mo, and an upper-normal BMI was a significant risk factor at 1mo. Low birth weight, child anomalies, and non-initiation of breastfeeding were possible mediators. Pregnant women with severe to moderate underweight, overweight, obesity, and upper-normal BMI are important targets for PD prevention, and possible mediators may indicate PD risk among high-risk groups.