Characteristics of the spontaneous milk ejecting activity occurring during human lactation

Continuous recordings of intramammary pressure performed in 158 normal lactating women, and 22 women during spontaneous labor were retrospectively analyzed in order to characterize spontaneous milk ejecting activity (SA). SA appeared as spurts of contraction waves as early as the first post-partum day and throughout the first two months of lactation. The spurts of SA differed from reflex milk ejection evoked by baby's suckling. It showed two different patterns: a) single contraction waves with a rapid rise time, a slow decay time, high amplitude and a short duration; b) multiple contraction waves with a longer rise and decay times, progressively decreasing intensity and a large duration depending on the number of contractions per spurt. The multiple pattern occurred in 34.2% (41/120) of patients at 2.7 +/- 0.1 days of post-partum and increased to 84.3% (59/70) at 34.5 +/- 1.2 days. Accordingly, duration and the area under contraction curves increased throughout lactation. Mammary gland sensitivity to exogenous oxytocin (OT) did not change during the 2 months studies. SA was significantly greater in full nursing mothers than in the partially nursing ones. The existence of a system for milk-ejection different from baby's suckling was further described. A neuroendocrine mechanism and the release of increasing amounts of OT are suggested as important factors in the control of this phenomenon.