A Relaxin-Defined Late-Luteal Endocrine Transition State in the Human Menstrual Cycle

Relaxin-2 shows a sharp, reproducible late-luteal peak in 80 natural menstrual cycles from 34 healthy women, peaking at cycle day 28.0 with ~4-day offset from progesterone.

• Why it matters: Relaxin’s nonpregnant physiology is poorly defined, and key reproductive hormones lack complete characterization across the natural menstrual cycle.
• What they did: Dense longitudinal endocrine profiling across 80 cycles used generalized additive modeling of cycle-normalized trajectories to compare relaxin-2 dynamics with estradiol and progesterone.
• The result: Progesterone peaked at day 23.9 while relaxin peaked at day 28.0, with lower inter-cycle variability and detectable relaxin into early menses; in combined oral contraceptive users, cyclic relaxin was effectively absent.