What Is Relaxin?

Portrait of young happy pregnant woman standing by the window
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As its name implies, relaxin is a hormone that helps "relax" a pregnant person's body during late pregnancy in preparation for childbirth.

The relaxin hormone also plays a significant role in the reproductive process. After ovulation, relaxin levels increase to prepare the uterus for a possible pregnancy. Your relaxin hormone levels will begin to drop if conception does not occur; but if you do conceive, relaxin hormone levels increase to help support the pregnancy.

Let's take a look at relaxin's role before and during pregnancy, as well as how it works with other hormones in your body to help produce and sustain new life.

Relaxin's Role in Your Menstrual Cycle

When many women think of their menstrual cycle, they think primarily of their period. However, your menstrual cycle is actually broken down into two phases, and your hormones are hard at work during both of them to prepare for a pregnancy.

The Follicular Phase

The first half of the menstrual cycle is known as the follicular phase. This phase begins on the first day your period starts and lasts until ovulation. Ovulation generally occurs around day 14, though this varies among women depending upon the typical length of their cycle.

The follicular phase refers to when your body releases hormones that promote egg growth and development on the ovaries. Once a mature egg develops, it is released by the ovary in hopes of being fertilized, which is a process known as ovulation.

Relaxin is not a major player during the follicular phase. The main hormones during this half of your menstrual cycle are the gonadotropin-releasing hormone (GnRH), the follicle-stimulating hormone (FSH), estrogen, and the luteinizing hormone (LH). What happens once the egg is released influences the role relaxin will play during the next phase of your cycle.

The Luteal Phase

This phase begins the day ovulation occurs and lasts until your next period begins. In most women, the luteal phase lasts between 12-16 days. During this time, your hormones work together to prepare your body for a possible pregnancy.

The egg that is released by the ovary leaves behind an empty follicle known as the corpus luteum. The corpus luteum begins releasing progesterone, which works together with relaxin to thicken your uterine lining to help implant the fertilized egg.

If sperm fertilizes the egg, your progesterone and relaxin levels will kick into high gear. Progesterone will help sustain early pregnancy until the placenta takes over—typically around 10 weeks gestation. Simultaneously, relaxin is secreted by the corpus luteum before later being released by the placenta.

In the event conception does not occur, relaxin and progesterone levels will gradually begin to drop. With these hormones no longer supporting the uterine lining, your body will prepare to shed it during your next period.

Relaxin in Early Pregnancy

Relaxin levels are at their highest during early pregnancy. During the first trimester, relaxin helps promote the embryo's implantation into the uterine wall; it also encourages the growth of the placenta.

Relaxin works to inhibit uterine contractions during early pregnancy to help prevent early childbirth. Relaxin is also responsible for preparing your organs for their new roles during pregnancy. This means regulating your heart and kidney functions to help these organs adjust to the increased demands that the fetus will place upon them during pregnancy.

A woman's blood volume may increase by 30% to 50% during pregnancy. Relaxin helps relax the blood vessels so your body can more easily circulate the blood to you and your baby.

Unfortunately, relaxin is partially to blame for some unpleasant early pregnancy symptoms, such as gastrointestinal issues. This is the result of the hormone causing reduced gut motion. Luckily, women find problems like constipation and indigestion dissipate as their pregnancies progress.

Relaxin in Later Pregnancy

While relaxin levels decrease after the first trimester, the hormone's role is equally important as you get closer to childbirth.

Relaxin helps your body prepare for labor and delivery by loosening the pelvis's ligaments, bones, and muscles as you get closer to the big day. This helps to ease the process of delivering your baby.

During the final stretch of your pregnancy, your cervix will begin to soften and open to prepare for labor. Relaxin helps this process along while also encouraging the rupturing of the membranes surrounding your baby. This is often referred to as your water breaking. However, your water may not break well into labor.

If you're feeling a bit off-balance as the months progress, it's not just your imagination. Relaxin's loosening qualities can make you feel less steady as time goes on, so be sure to use caution as your center of gravity changes.

Complications From Relaxin

Chances are, you won't notice any severe side effects from relaxin, but some women do suffer from the impact the hormone has on their bodies. An estimated 1 in 300 pregnancies will experience symphysis pubis dysfunction, which can be described as severe pelvic pain.

Symptoms of Symphysis Pubis Dysfunction Include:

  • Pain in the lower back and thigh area
  • Difficulty navigating the stairs
  • Pain in the center of the pelvic region
  • Problems changing sleep positions

Relaxin can also cause the spine to loosen excessively in some pregnancies, causing discomfort. Wearing a support band can help alleviate symptoms, but your doctor or midwife should be able to advise you if further treatment is needed to control this condition.

A Word From Verywell

Your menstrual cycle is broken down into two phases, and your hormone levels fluctuate based on where you are in your cycle. Relaxin is most prevalent during the second half of your cycle—or the luteal phase—when levels rise to prepare your body for a possible pregnancy.

If you conceive, relaxin serves a new purpose in your body. The hormone pairs with progesterone to help sustain early pregnancy. As you get closer to delivery, relaxin begins preparing your body for childbirth by loosening your joints, ligaments, and bones to make the process easier.

Most women do not experience noticeable side effects related to relaxin during pregnancy. Speak to your doctor about any pelvic pain that feels abnormal, as this could signify complications that require specialized treatment.

4 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Bull, J.R., Rowland, S.P., Scherwitzl, E.B. Real-world menstrual cycle characteristics of more than 600,000 menstrual cyclesnpj Digit. Med. 2, 83 (2019) doi:10.1038/s41746-019-0152-7

  2. Reed BG, Carr BR. The Normal Menstrual Cycle and the Control of Ovulation. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.

  3. Hytten F. Blood volume changes in normal pregnancy. Clin Haematol. 1985 Oct;14(3):601-12. PMID: 4075604.

  4. Seidman AJ, Siccardi MA. Postpartum Pubic Symphysis Diastasis. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537043/

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By Renee Plant
Renee Plant is a health and wellness freelance writer with a passion for delivering well-researched, factual content to readers.