Relaxin is a small peptide hormone that plays multiple roles in preparing for and maintaining pregnancy. Early in gestation, the corpus luteum, a structure formed temporarily in the ovary, serves as the primary source of relaxin. As the pregnancy advances, the placenta and the decidua—the uterine lining—take over the majority of relaxin production. Its concentrations fluctuate widely, typically peaking during the first trimester, dipping in the second, and then often rising again toward the end of the third trimester.
Systemic Effects on the Mother’s Body
Beyond its mechanical functions, relaxin significantly influences the cardiovascular system and renal function. Pregnancy requires a substantial increase in blood volume and cardiac output to supply the growing fetus, and relaxin helps the body manage this demand. The hormone acts as a potent vasodilator, causing blood vessels to relax and widen, which increases blood flow throughout the body.
Systemic vasodilation decreases peripheral vascular resistance, allowing the heart to pump the increased blood volume more efficiently. Specifically, relaxin targets the renal arteries, increasing blood flow to the kidneys. By enhancing renal perfusion, the hormone aids in elevating the glomerular filtration rate (GFR), a measure of kidney function. This adjustment is necessary to process the higher fluid volumes of pregnancy and maintain appropriate electrolyte balance.
Preparing the Pelvis and Joints
One of the most noticeable effects of relaxin relates to the musculoskeletal system, where it facilitates the remodeling of connective tissue. The hormone acts on fibroblasts, cells responsible for producing collagen, to modify the structure of ligaments and tendons. This action promotes the softening and lengthening of these tough, fibrous tissues to increase flexibility.
The primary target of this remodeling is the pelvic girdle, specifically the ligaments surrounding the pubic symphysis and sacroiliac joints. By softening these attachments, relaxin allows the pelvis to become more pliable, slightly increasing the diameter of the birth canal in preparation for labor. This increased laxity provides the flexibility necessary for the fetus to pass through the pelvis during delivery.
While beneficial for birth, this hormone-induced joint laxity can sometimes lead to instability and discomfort for the mother. Increased movement in the pelvic joints can contribute to pelvic girdle pain (PGP). This instability arises because the ligaments become hypermobile, leading to pain when walking or shifting position. However, the degree of joint movement is variable, and not everyone experiences significant pain or instability despite the presence of the hormone.
Role in Uterine Environment and Labor
Relaxin performs a dual function related to the uterus, first promoting pregnancy maintenance and later assisting in preparation for birth. During the early and middle stages of gestation, relaxin helps maintain myometrial quiescence, which is the state of uterine relaxation. It prevents the smooth muscle of the uterus from contracting prematurely, reducing the risk of preterm labor.
As the pregnancy nears term, the hormone’s focus shifts to facilitating the final preparations for delivery, specifically through cervical ripening. Cervical ripening involves the softening, thinning, and effacement of the cervix, which is necessary before true labor contractions can be effective. Relaxin promotes this process by altering the collagen and connective tissue matrix within the cervical structure.
The hormone breaks down the dense collagen bundles that keep the cervix firm and closed. This remodeling allows the tissue to become more hydrated and compliant, making the cervix ready to dilate when labor begins. This change from a firm, closed structure to a soft, pliable one is a non-contractile process that is distinct from the muscle contractions of the uterus itself.