Relaxin is a peptide hormone structurally related to insulin. It is temporarily produced in high concentrations during pregnancy to prepare the mother’s body for the physical demands of carrying a fetus and giving birth. Its main action involves modulating connective tissue and influencing systemic processes. Relaxin is fundamental to the body’s ability to stretch and accommodate the developing baby, facilitating crucial changes for a successful pregnancy.
The Source and Timing of Relaxin
Relaxin production is primarily initiated by the corpus luteum, a temporary endocrine gland that forms in the ovary after ovulation. If conception occurs, this structure continues to secrete the hormone into the bloodstream during early pregnancy. As gestation advances, the placenta and the decidua (the specialized lining of the uterus) also contribute to circulating relaxin levels.
Relaxin levels follow a distinct pattern throughout gestation, showing two main peaks. The highest concentration occurs during the first trimester, typically peaking around 8 to 12 weeks. A secondary, smaller surge is observed just before delivery, which assists with final preparations for labor. This early peak is important for establishing the pregnancy and preventing premature uterine contractions.
Preparing the Body for Birth: Ligament Laxity
The most recognized function of relaxin involves remodeling connective tissue to prepare the body for childbirth. The hormone acts by influencing collagen, the primary protein component of ligaments and tendons, specifically by activating enzymes like collagenase. This process leads to the softening and stretching of ligaments throughout the body, resulting in joint laxity.
The effects are most pronounced in the pelvic girdle, affecting the pubic symphysis and the sacroiliac joints. By increasing the elasticity of these ligaments, relaxin allows the pelvic bones to separate and widen slightly. This increased mobility is mechanically necessary to enlarge the birth canal, permitting the baby’s passage during vaginal delivery.
Systemic Roles Beyond Joint Loosening
Beyond its mechanical effects on joints, relaxin performs several other systemic functions that sustain the pregnancy. In the cardiovascular system, the hormone helps manage the dramatic increase in blood volume, which can be up to 50% higher than pre-pregnancy levels.
It achieves this by promoting vasodilation (the relaxation and widening of blood vessels), which lowers peripheral resistance and increases overall cardiac output. Relaxin also regulates the renal system to accommodate the increased metabolic demands of pregnancy. It increases renal blood flow and the glomerular filtration rate, allowing the kidneys to efficiently process and excrete waste products from both the mother and the fetus.
During early pregnancy, relaxin inhibits the contractility of the smooth muscle in the uterine wall. This action is important for preventing premature contractions, thereby helping to maintain the pregnancy.
Practical Strategies for Managing Hormone-Induced Instability
The ligament laxity induced by relaxin, while necessary for birth, can cause instability and discomfort, particularly pelvic girdle pain. To manage this instability, pregnant individuals should focus on strengthening and stabilization rather than stretching. It is recommended to avoid activities that stress the loosened joints, such as heavy lifting, excessive stair climbing, or standing on one leg.
Supportive devices can provide external stability to the pelvis and lower back. Wearing a pregnancy support belt or a belly band helps stabilize the joints and alleviate pain during movement. Maintaining good posture and incorporating gentle, low-impact exercises like swimming or prenatal yoga can strengthen the surrounding muscles without overstressing the ligaments. When resting, placing a pillow between the knees while sleeping helps maintain proper pelvic alignment and reduce discomfort.