Relaxin is a hormone that belongs to the same family as insulin, built from two protein chains linked together by chemical bonds. It is best known for loosening ligaments and softening the cervix during pregnancy, but it also plays roles in blood vessel function, kidney filtration, and male fertility. Your body produces it in small amounts outside of pregnancy too, making it more versatile than its name suggests.
Where Relaxin Comes From
In women, the ovaries secrete relaxin during the second half of each menstrual cycle. If pregnancy occurs, the placenta joins in and begins producing it as well. Levels climb quickly, peaking near the end of the first trimester at roughly 0.9 to 1.0 ng/mL in the blood for singleton pregnancies. From there, concentrations gradually taper through the second and third trimesters, though they never disappear entirely until after delivery.
Men produce relaxin too. The prostate gland secretes it into seminal fluid at concentrations roughly 60 times higher than what’s found in male blood. This local production hints at a direct role in sperm function rather than a systemic one.
How It Loosens Joints and Ligaments
Relaxin’s most familiar job is making connective tissue more flexible. It does this by triggering enzymes that break down collagen, the structural protein that gives ligaments, tendons, and cartilage their stiffness. Specifically, it activates a cascade of protein-degrading enzymes that disassemble the extracellular matrix, the scaffolding that holds connective tissue together.
This softening effect is most important in the pelvis. During pregnancy, the pubic symphysis (the joint at the front of the pelvis) and the sacroiliac joints need to become more flexible to allow a baby to pass through. Relaxin helps make that possible. But the effect isn’t limited to the pelvis. Studies on human anterior cruciate ligaments (ACLs) show that tissue treated with relaxin has reduced structural integrity. In animal experiments, rabbits given relaxin developed significantly weaker ACLs and measurably more knee laxity compared to untreated controls. Relaxin also decreases knee cartilage stiffness and reduces tendon rigidity by promoting collagen breakdown within those tissues.
This widespread loosening is why many pregnant women notice joint instability not just in their hips, but in their knees, ankles, and feet. It also helps explain why some women go up half a shoe size during pregnancy, as ligaments in the feet relax and the arches flatten slightly.
Effects on Blood Vessels and the Heart
Relaxin is a potent vasodilator, meaning it widens blood vessels and makes artery walls more flexible. During the first trimester, as circulating relaxin levels rise, several cardiovascular changes happen in parallel: cardiac output increases (the heart pumps more blood per minute), arterial compliance rises (blood vessels stretch more easily), and systemic vascular resistance drops (the overall “tightness” of the blood vessel network decreases). The heart achieves this higher output mainly by increasing stroke volume, pushing more blood with each beat rather than beating faster.
In animal studies, relaxin’s ability to lower vascular resistance is proportional to how elevated resistance is at baseline. In other words, the higher the starting tension in the blood vessels, the more relaxin brings it down. This property caught the attention of heart failure researchers, since patients with acute heart failure often have dangerously high vascular resistance. A synthetic version of relaxin called serelaxin went through large clinical trials for acute heart failure, with a 6,800-patient trial (RELAX-AHF-2) representing the biggest effort. Early data suggested it could improve markers of organ damage in heart failure patients regardless of whether their hearts had preserved or reduced pumping function.
Kidney Function and Blood Flow
The kidneys are particularly responsive to relaxin. By relaxing the small arteries that feed the kidneys, relaxin increases renal blood flow and filtration rate during pregnancy. This is one reason pregnant women need to urinate more frequently, even in the first trimester before the uterus is large enough to press on the bladder. The kidneys are simply filtering more blood.
In a study of healthy volunteers given an intravenous infusion of relaxin at pregnancy-level concentrations for just five hours, renal blood flow jumped by 43%. In patients with scleroderma (a condition that can damage small blood vessels, including those in the kidneys), 26 weeks of relaxin treatment increased the kidney’s filtering capacity by 15% to 20%. These findings suggest relaxin’s kidney effects are not unique to pregnancy but are a fundamental property of the hormone.
Preparing the Body for Labor
In the final stages of pregnancy, relaxin helps ripen the cervix, softening and thinning it so it can dilate during labor. Clinical trials that compared relaxin to a placebo found that cervixes treated with relaxin were significantly more likely to become favorable for labor: only about 22% remained unchanged after treatment, compared to 49% in the placebo group.
Interestingly, relaxin may slightly inhibit uterine muscle contractions, which sets it apart from other labor-induction agents that can sometimes cause excessive uterine stimulation. However, it doesn’t appear to reduce the overall need for oxytocin (the hormone commonly used to strengthen contractions) once labor is underway. Its contribution seems to be specifically about getting the cervix ready rather than driving the contractions themselves.
Relaxin’s Role in Male Fertility
Although relaxin is often described as a pregnancy hormone, it has meaningful effects on sperm. The relaxin receptor is present on human sperm cells, concentrated near the head of the sperm. In laboratory studies, relaxin slowed the natural decline in sperm motility over time, kept mitochondrial activity higher (mitochondria power the sperm’s tail), and reduced the rate of programmed cell death. It also increased sperm hyperactivation, a vigorous swimming pattern needed to penetrate an egg, and promoted the acrosome reaction, a chemical release from the sperm head that allows it to fuse with the egg.
Relaxin concentrations in seminal fluid average about 10.3 nM, compared to just 0.16 nM in male blood. This dramatic difference confirms that the prostate produces relaxin specifically for the reproductive tract, where it appears to help sperm survive longer and fertilize more effectively. These properties have led researchers to consider relaxin as a potential additive in assisted reproduction procedures.
Pelvic Pain and Relaxin: A Complicated Link
Many people assume that high relaxin levels directly cause the pelvic girdle pain that affects a significant number of pregnant women. The logic seems straightforward: more relaxin means looser joints, which means more instability and pain. But the evidence is surprisingly mixed. A systematic review of six studies found that four of them, including three of the four highest-quality studies, could not find a positive association between relaxin levels and pregnancy-related pelvic girdle pain.
One well-designed prospective study did find that women with pain at the pubic symphysis and hip region had statistically higher relaxin levels. So a connection likely exists in some women, but relaxin levels alone don’t reliably predict who will develop pelvic pain. Other factors, including pre-existing joint stability, muscle strength, posture, and the mechanical load of carrying a growing baby, appear to matter just as much or more. This means that two women with identical relaxin levels can have very different experiences with pelvic comfort during pregnancy.