Serum Relaxin Quantity and Timing

Certainty Style Key
Hover over keys for definitions:
True   Likely   Speculative
Human Uniqueness Compared to "Great Apes": 
Likely Difference
MOCA Topic Authors: 

Relaxin is a peptide hormone. The main source of circulating relaxin is the corpus luteum of the ovary. Humans are unusual among mammals, but not unique among primates, in that relaxin is detectable in the luteal phase of non-conception cycles. Unlike rodents, guinea pigs and pigs, there is no near-term increase or surge in serum relaxin in human or non-human primates.

In the non-conception ovarian cycle of anthropoid primates, serum relaxin concentration peaks about 10 – 12 days after ovulation. Data are available for the following species: human, chimpanzee, baboon, rhesus macaque, cynomolgus macaque (where peak luteal-phase level is reported at 16 days post-ovulation), and common marmoset (Hayes, 2004).

If fertilization occurs, relaxin rises to a peak in the first third of pregnancy, then declines slightly and stabilizes for the remainder of pregnancy in humans, chimpanzees, baboons and macaques. Chimpanzees may have absolutely higher concentrations of circulating relaxin in pregnancy as well as during the luteal phase of the cycle (Steinetz et al., 1992). In marmosets the rise in serum relaxin during pregnancy does not peak until mid-gestation (Einspanier et al., 1999). Relaxin was originally isolated from pigs. In this species as well as guinea pigs and rodents, relaxin secretion peaks at the end of gestation and the physiological role of relaxin appears related to softening of the cervix and relaxation of pelvic ligaments both of which facilitate passage of the fetuses. Exogenously administered relaxin has a similar affect on women, however, since there is no endogenous rise in circulating relaxin near-term the physiological role of relaxin in human and non-human primate parturition is uncertain.

There are three genes (H1, H2, H3) that encode relaxin in humans (Hayes, 2004). Circulating relaxin, produced by the ovary, is primarily the result of transcription of H2. In the great apes (chimps, gorillas, and orangutans) there are two genes, equivalent to H1 and H2. H1 and its homologue in the great apes probable arose as a duplication event of H2. In baboons, macaques, and marmosets there is one gene that encodes relaxin, it is equivalent to H2. An H3 equivalent has not been identified in non-human primates but is known from the mouse (M3). H3 is expressed only in the brain.




  1. Biology of primate relaxin: a paracrine signal in early pregnancy?, Hayes, E. S. , Reprod Biol Endocrinol, 06/2004, Volume 2, p.36, (2004)
  2. Relaxin regulation of endometrial structure and function in the rhesus monkey., Goldsmith, Laura T., Weiss Gerson, Palejwala Smita, Plant Tony M., Wojtczuk Andrea, W Lambert Clark, Ammur Nael, Heller Debra, Skurnick Joan H., Edwards Dean, et al. , Proc Natl Acad Sci U S A, 2004 Mar 30, Volume 101, Issue 13, p.4685-9, (2004)
  3. Molecular remodeling of members of the relaxin family during primate evolution., Klonisch, T, Froehlich C, Tetens F, Fischer B, and Hombach-Klonisch S , Mol Biol Evol, 03/2001, Volume 18, Issue 3, p.393-403, (2001)
  4. Relaxin in the marmoset monkey: secretion pattern in the ovarian cycle and early pregnancy., Einspanier, A, Nubbemeyer R, Schlote S, Schumacher M, Ivell R, Fuhrmann K, and Marten A , Biol Reprod, 08/1999, Volume 61, Issue 2, p.512-20, (1999)
  5. Expression of human relaxin genes: characterization of a novel alternatively-spliced human relaxin mRNA species., Gunnersen, J M., Fu P, Roche P J., and Tregear G W. , Mol Cell Endocrinol, 04/1996, Volume 118, Issue 1-2, p.85-94, (1996)
  6. Variations in serum relaxin (hRLX-2) concentrations during human pregnancy., Petersen, L K., Vogel I, Agger A O., Westergård J, Nils M, and Uldbjerg N , Acta Obstet Gynecol Scand, 04/1995, Volume 74, Issue 4, p.251-6, (1995)
  7. Characterization of two relaxin genes in the chimpanzee., Evans, B A., Fu P, and Tregear G W. , J Endocrinol, 03/1994, Volume 140, Issue 3, p.385-92, (1994)
  8. Relaxin, Sherwood, O.D. , The Physiology of Reproduction, Volume 1, New York , p.861-1009, (1994)
  9. Measurement of periimplantational relaxin concentrations in the macaque using a homologous assay., Stewart, D R., Stouffer R, Overstreet J W., Hendrickx A, and Lasley B L. , Endocrinology, 1993 Jan, Volume 132, Issue 1, p.6-12, (1993)