Pre-pregnancy body mass index and lactational mastitis: The Japan Environment and Children's Study

Purpose: The association between maternal physique and prevalence of lactational mastitis (LM) in a large study population has not been reported. In this study, we aimed to evaluate the association between pre-pregnancy body mass index (BMI) and LM prevalence.

Methods: We analyzed data collected from participants enrolled in the Japan Environment and Children's Study, a nationwide birth cohort study conducted between 2011 and 2014. Data from women with singleton births at and after 22 weeks of gestation were analyzed. LM occurrence was identified from medical record transcripts obtained 1-month post-childbirth. Based on the pre-pregnancy BMI, participants were categorized into Group 1 (< 18.5 kg/m²), Group 2 (18.5-24.9 kg/m²), and Group 3 (≥ 25.0 kg/m²). Excluding participants who were not breastfeeding, multivariable logistic regression models were used to evaluate the odds ratios (ORs) for LM in women who were breastfeeding in each BMI category. Group 2 was designated as the reference group. Maternal demographic and socio-economic characteristics were used as confounding factors.

Results: We analyzed 83,170 participants. The adjusted OR for LM in Group 3 was 0.752 (95% confidence interval, 0.606-0.932). The adjusted OR for LM in Group 1 was not significantly changed.

Conclusions: High pre-pregnancy BMI was associated with decreased LM prevalence compared with that in the reference group. The maternal preconception physique may be associated with LM prevalence.