Background/Objectives: Optimal infant growth is reliant on the sufficient intake of human milk. Studies in animal models speculate that multiparous mothers produce a higher milk yield compared to primiparous mothers. We aimed to examine if there are relationships between consecutive lactations and infant demographics and both maternal 24 h milk production (MP) and infant milk intake (MI). Methods: Lactating mothers 1–6 months postpartum (n = 36; 25 fully breastfeeding (FBF), 11 supplementing with commercial milk formula (partly breastfeeding (PBF)) test-weighed their infants for 24 h during two consecutive lactations and provided demographic information. Twenty-four-hour MP by breast, infant MI (including mothers’ own expressed milk and formula), breastfeeding and expressing frequencies were measured. The statistical analysis used linear mixed modelling accounting for infant birth weight (FBF) or time postpartum (PBF) and for the random effect of the participant. Results: In the FBF group, there were no differences between lactations in terms of MP (p = 0.31) or the infant mother’s own MI (p = 0.14). The birth weight was higher for consecutive lactation infants (p = 0.008). Infant sex was not associated with MP (p = 0.12) or the infant mother’s own MI (p = 0.090). In the PBF group, the breastfeeding frequency (p = 0.042), MP (p = 0.025) and infant mother’s own MI (p = 0.019) were higher in consecutive lactations whilst formula intake was lower (p = 0.004). Conclusions: This study suggests that in fully breastfeeding women, there is no significant effect of consecutive lactation or infant sex on MP or infant MI during established lactation.

Optimal infant growth is reliant on both the production and intake of sufficient human milk. Some studies, in particular animal models, suggest that multiparous mothers produce a higher yield of milk compared to primiparous mothers. The aim of this study was to examine whether there is a relationship between successive lactations and maternal 24 h milk production and infant milk intake. Lactating mothers who did not feed commercial milk formula (n = 22) measured their milk production at 1–6 months postpartum by test-weighing their infants for 24 h during two consecutive lactations (L1: at 3.0 ± 1.2 months, L2: at 2.6 ± 1.0 months; (p = 0.26)) and provided the dyad’s demographics. Twenty-four-hour milk production by breast, infant 24 h milk intake (including mothers’ own expressed milk), and breastfeeding and expressing frequencies were measured. Statistical analysis used linear mixed modelling accounting for infant birth weight and the random effect of participant. There were no differences between L1 and L2 for milk production (L1: 748 ± 122 g; L2: 768 ± 157 g; p = 0.57), infant milk intake (L1: 744 ± 133 g; L2: 776 ± 189 g; p = 0.50), 24 h breastfeeding frequency (L1: 13 ± 4; L2: 12 ± 3; p = 0.28), and expression frequency (L1: 1.4 ± 1.9; L2: 1.4 ± 2.8; p = 0.95). Birth weight was higher with the successive lactation (L1: 3260 ± 345 g; L2: 3509 ± 237 g; p = 0.002). Infant sex was not associated with 24 h milk production (p = 0.21), milk intake (p = 0.62), or breastfeeding frequency (p = 0.17). The findings of this study suggest that in humans there is no effect of successive lactations or infant sex on 24 h milk production or infant milk intake.