Characteristics Associated with Infant Feeding with Both Breast Milk and Formula Milk

Background: Breastfeeding benefits mothers and infants, and the promotion of breastfeeding is important. Feeding strategies include exclusive breastfeeding, feeding with both human milk and formula, and exclusive formula feeding. Objectives: This study was conducted to clarify the actual situation by assuming that mixed feeding comprises several groups with different characteristics. At the same time, the study also aimed to clarify the factors associated with breastfeeding. Methods: Single-term infants without underlying disease born at Tokyo Metropolitan Hospital between 2019 and 2024 participated in this study. The distribution of formula intake among infants receiving both human milk and formula was analyzed using a Gaussian mixture model, and the optimal number of distribution components was calculated using the Bayesian information criterion. Using linear regression analysis, factors associated with formula intake were identified. Results: A total of 2628 participants (exclusive breastfeeding, 842 (32.0%); mixed feeding with human milk and formula, 1496 (56.9%); and exclusive formula feeding, 290 (11.0%)) were included in the study. Linear regression analysis showed that the factors associated with amount of formula intake were late preterm birth (coefficient 39.7, p < 0.01), maternal age (reference under 30 y, age ≥ 30 y and <35 y coefficient 6.3, p = 0.66, age ≥ 35 y and <40 y coefficient 45.5, p < 0.01, age ≥ 40 y coefficient 106.9, p < 0.01) and delivery mode (cesarean section, coefficient 53.6, p < 0.01). Conclusions: Feeding strategies involving both human and formula milk are not homogeneous, and interventions should be developed based on these differences. Moreover, several factors were found to be associated with breastfeeding, which may help promote breastfeeding.