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

A bstract Dynamic endocrine transitions govern female physiology, yet several reproductive hormones remain incompletely characterized across the natural menstrual cycle. Relaxin, a peptide hormone classically associated with pregnancy, has recognized vascular, extracellular matrix, and anti-fibrotic actions, but its physiology in nonpregnant women remains poorly defined. We performed dense longitudinal endocrine profiling across 80 natural menstrual cycles from 34 healthy women, complemented by oral contraceptive users, to define circulating relaxin-2 dynamics relative to ovarian steroids. Using generalized additive modeling of cycle-normalized trajectories, we identified relaxin as a highly phase-locked late-luteal hormone with a sharp, reproducible peak distinct from estradiol and progesterone. Progesterone rose earlier after ovulation and peaked at cycle day 23.9, whereas relaxin peaked significantly later at day 28.0, revealing an approximately 4-day temporal offset. Relaxin showed lower inter-cycle variability and narrower temporal dispersion than progesterone, indicating unexpectedly precise endocrine timing. Detectable relaxin persisted into early menses, suggesting continued luteal peptide release across the menstrual boundary. In combined oral contraceptive users, endogenous cyclicity was attenuated and circulating relaxin was effectively absent despite exogenous progestin exposure, implicating ovulation-dependent corpus luteum biology rather than steroid exposure alone as the primary determinant of relaxin secretion. These findings identify relaxin as a reproducible component of normal cyclic endocrine physiology rather than a hormone restricted to pregnancy. The discrete late-luteal relaxin window establishes a previously under-resolved temporal feature of the menstrual cycle and provides a framework for investigating its reproductive, vascular, and connective-tissue significance.