Human endometrium contains relaxin that is progesterone-dependent.

The avidin-biotin immunoperoxidase method using two anti-porcine relaxin antisera was employed to study the occurrence of relaxin in the endometrium of 102 pre- or postmenopausal women. Relaxin was not found in the endometrium in the following conditions: 1. normal proliferative phase (n = 27), 2. cystic glandular hyperplasia (n = 12), 3. early secretory endometrium during 1-3 postovulatory days (n = 12), 4. atrophic endometrium from postmenopausal women not taking hormones (n = 7), and 5. atrophic or proliferative endometrium of women undergoing estrogen replacement therapy (n = 5). Relaxin was invariably present 1. in the secretory endometrium from natural cycles after day 3 postovulation (n = 28), 2. in the secretory endometrium of previously unovulatory premenopausal women taking progestogens (n = 6), and 3. in the secretory endometrium during the latter part of the cycle of postmenopausal women undergoing estrogen-progestogen replacement therapy (n = 5). Our results indicate that the occurrence of relaxin in the endometrium is progesterone-dependent, but the corpus luteum is not required for relaxin synthesis to occur in non-pregnant women. The timing of the appearance of relaxin in the endometrium during natural cycles coincides with implantation, thus suggesting a role for relaxin in the early events of human reproduction.