Breast Milk Expression Frequency and Production, Na Concentrations and Na:K Ratios in the First 4 Weeks After Preterm Birth

Background/Objectives: Low milk production is more prevalent after preterm birth and may be associated with infrequent milk expression, delayed secretory activation and elevated milk biomarkers including sodium (Na) and sodium–potassium ratio (Na:K). This study aimed to describe milk production, expression frequency, and milk biomarkers (Na and Na:K) in the first 4 weeks and explore associations in the first 2 weeks after preterm birth. Methods: Women who birthed at 28–34 weeks of gestation provided milk expression data and milk samples every second day from birth to Day 10, then every third day until infant transfer/discharge from the neonatal unit. Lactation characteristics and milk Na and Na:K across the first 4 weeks were described, and associations between milk production, expression frequency and milk biomarkers were examined. Results: In a sample of N = 44 women that maintained a median expression frequency of 6–7 × 24 h, temporal patterns in milk Na and Na:K were similar to those observed after birth at term, and a milk production ≥ 600 mL/24 h was achieved by 61.5% on Day 13. One third of women experienced delayed secretory activation. Expression volumes on Day 4 were associated with milk production on Day 13 and Day 16 (both p < 0.001). Conclusions: Our findings suggest that low expression volumes in the days after preterm birth may indicate women at risk of low milk production. Further research is needed to determine the predictive value of early expression frequency and milk composition on subsequent milk production.