Abstract
Relaxin is a 6-kDa member of the relaxin family of peptide hormones, similar in structure to insulin and insulin-like growth factors. In pregnancy, relaxin circulates in the blood of all mammals, although the source and pattern of relaxin secretion are species–specific. During the cycle and in early pregnancy, relaxin actions include ovarian follicular growth and remodeling, implantation and decidualization, and myometrial quiescence. Later in pregnancy, a role for relaxin has been proposed in uterine growth, remodeling the birth canal and fetal membranes in preparation for parturition, mammary gland development, and changes in maternal vasculature.