BackgroundExcessive weight loss in newborns is associated with neonatal complications such as jaundice and dehydration, which cause renal failure, thrombosis, hypovolemic shock, and seizures. The identification of the risk factors for excessive weight loss will help to discover preventive measures. The aim of this study was to determine the factors associated with excessive weight loss, defined as weight loss of ≥10%, in breastfed full-term newborns in Japan.MethodsThe present retrospective study, which was performed in a tertiary perinatal center accredited as a Baby-Friendly Hospital, included neonates who were born alive with a gestational age of ≥37 weeks. Cases of multiple births, admission to the neonatal intensive care unit (NICU), referral to another facility, or exclusive formula feeding were excluded. Multivariate logistic regression analyses were performed to assess the association between maternal or neonatal characteristics and excessive weight loss.ResultsWe studied 399 newborns, of whom 164 (41%) had excessive weight loss. According to the adjusted multiple regression analysis, the factors associated with excessive weight loss were an older maternal age, primiparity, and antepartum Caesarean section, with adjusted odds ratios (95% Confidence Intervals [CIs]) of 1.07 (1.02, 1.11), 2.72 (1.69, 4.38), and 2.00 (1.09, 3.65), respectively.ConclusionsClose monitoring of infants born to older mothers, primiparous mothers, or infants delivered by antepartum Cesarean section is recommended, and earlier supplementation with artificial milk may be considered.