Purpose
This study aimed to evaluate the effect of delayed lactogenesis Ⅱ on early milk volume in mothers expressing milk for their preterm infants.

Methods
142 mothers with preterm infants participated in a longitudinal cohort study, the milk volumes over 14 days postpartum between mothers with delayed lactogenesis Ⅱ (≥ 72 hours) and mothers with non-delayed lactogenesis Ⅱ(< 72 hours) were compared using Wilcoxon's rank sum tests.

Results
The prevalence of delayed lactogenesisⅡ among mothers of preterm infants was 36.0% (36/100). There existed negative correlations between the onset of lactogenesis Ⅱ and the daily milk volumes( rs = −0.525∼−0.354, p = .002 ∼ p < .001). The milk volumes in every 24-hour of the 14 days postpartum in delayed group were significantly less than that in non-delayed group (p = .002 ∼ p < .001). After controlling for the covariates, pregnancy-induced hypertension syndrome, delayed expression initiation, shorter daily sleeping time were found to be the risk factors for delayed lactogenesis Ⅱ.

Conclusion
Delayed lactogenesis Ⅱ was associated with lower milk volume in early postpartum period. Women who were at risk for delayed lactogenesis Ⅱ need targeted interventions and additional support during pregnancy and postpartum.