Human Relaxin-2 is synthesized and secreted as a preprohormone that is 18 kDa in size and 185 amino acids in length. It is a peptide hormone best known for its important physiological roles during pregnancy. The role of relaxin in pregnancy physiology differs considerably between mammals. In humans and other primates, relaxin does play a role in pregnancy, albeit an earlier role of modulating endometrial physiology during implantation. In rodents there is a major pre-labor surge in circulating relaxin levels, corresponding to its role in parturition; however, in humans and other primates, the serum levels of relaxin are highest during the first trimester. In women, the ovarian corpus luteum is the source of circulating Relaxin-2. It should be noted that Relaxin-2 is also present in males and is synthesized by non-reproductive organs such as the kidney, heart, and lungs suggesting a more global contribution for this hormone. Relaxin-2 initiates its pleiotropic effects through multiple pathways on a variety of cell types. It confers activity by binding to the leucine-rich G protein-coupled receptor LRG7, and with significantly less affinity to LRG8.