relaxin

Relaxin is a member of the insulin-like peptide hormone family, structurally similar to insulin and IGF, secreted mainly by the corpus luteum of pregnancy, with additional contributions from the decidua, placenta, and chorion. Its levels rise during pregnancy, peaking in the first trimester and again near term.

Physiologic actions: - Promotes remodeling of collagen in the pubic symphysis and pelvic ligaments, increasing pelvic joint flexibility to facilitate vaginal delivery. - Softens and dilates the cervix (cervical ripening) by increasing collagenase activity and altering extracellular matrix composition. - Inhibits myometrial contractility, helping maintain uterine quiescence during pregnancy (works with progesterone). - Has vasodilatory effects, contributing to the increased renal blood flow and glomerular filtration rate seen in pregnancy; also implicated in the physiologic hydronephrosis of pregnancy. - May play a role in mediating the increased cardiac output and decreased systemic vascular resistance of pregnancy.

Clinical relevance: - Used as a biomarker in some pregnancy tests and studied in luteal function assessment. - Investigated as a therapeutic agent in heart failure due to its vasodilatory and anti-fibrotic properties (e.g., serelaxin trials), though not yet a mainstay therapy. - Levels are essentially undetectable in non-pregnant women outside the luteal phase, so measurement can help confirm ovarian luteal activity. - Understanding its role helps explain physiologic changes of pregnancy tested on boards, such as increased GFR, hydronephrosis of pregnancy, and pelvic girdle pain/laxity.