Immunoreactive relaxin surge in the peritoneal fluid of women during the midluteal phase.

Relaxin was assayed in the peritoneal fluid (PF) of 176 women with normal ovulatory cycles. Validation of the assay in this fluid was carefully established. Relaxin was rarely detected before day 20 of the cycle. From days 21 to 24, all samples contained detectable concentrations of the peptide, ranging between 75 and 775 pg/ml. Apparition of relaxin in the PF was not directly related to the ovulatory follicular rupture. It was also delayed, compared with the postovulatory rise of plasma and peritoneal progesterone. Relaxin is thus present in the PF only during the period propitious to human embryo implantation.