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The Metamorphosis of Your Mightiest Muscle

There is no other organ quite like the uterus. If men had such an organ they would brag about it. So should we.”

– Ina May Gasken

Let’s talk about the uterus.

All of us lived inside one at some point of our lives. Many of us have the great luck to use ours to do incredible things—like grow and birth a new human!

During pregnancy, especially, this organ is a superhero. She grows/stretches at an incredible rate—expanding from the size of an nectarine to a watermelon over the 9 1/2 months of pregnancy. And she is super strong—the most powerful muscle in the human body.

uterine contractions uterus expansion

But her strength and ability to quadruple in size is just the beginning of the pregnant uterus’ superpowers. The role and responsibilities of the uterus also changes dramatically within a tiny window of time. And how that transformation happens is nothing short of remarkable.


Let’s start at the beginning…

For 99% of her pregnant days, the uterus is a nurturer. To start, she must let a tiny ball of cells nestle into her fleshy wall and set up shop. Then, under the careful watch of a variety of hormones and other factors she  creates and maintains a peaceful, expanding sanctuary for that growing ball of cells until baby is ready for the world. Throughout this “growing phase” (nectarine → watermelon), the uterine muscle must remain calm, cool, and collected.

But at the end of your baby’s tenure, things change in a matter of hours to days. The mighty muscle finally gets to show off her power and strength and expel that baby into the world.


Uterine contractions get the baby and the body to the edge of the cervical cliff, ready for that final “push.”


With birth, mom usually gets all the glory (as is her due), with every Hollywood birth scene focusing on the drama of pushing the baby out. But the uterus is mom’s underappreciated  sidekick. Uterine contractions get the baby and the body to the edge of the cervical cliff, ready for that final “push.” In fact, studies have suggested that letting uterine contractions do all the heavy lifting might be just as effective as mom bearing down[1].


Uterine contractions and the metamorphosis of a muscle.

The uterine transformation at the very end of pregnancy is remarkable.

The uterus has multiple jobs. Multiple, mutually exclusive jobs: from sanctuary and nurturer to powerful expulsion machine. This switch happens in a matter of days. Switching from one job to the other in such a short time span requires fast physiological changes, mostly within the uterine muscle, also known as the myometrium.

The myometrium is made up of individual muscle cells, called myocytes. In the early days of pregnancy, these independent cells contract and flex in isolation. Sometimes they join their neighbor myocytes in the effort or even collaborate in small muscle clusters.


The difference between Braxton Hicks contractions and true labor-inducing contractions lies in how those myocytes work together.


Isolated feats of myocyte flexing will not kick out the growing little human. The mom-to-be might experience the flexing as a twinge here or there (those pesky Braxton Hicks contractions). These sporadic moments of uterine contractions are no cause for concern, just a natural part of pregnancy, a warm-up for a big physical event.

The difference between Braxton Hicks contractions and true labor-inducing contractions lies in how those myocytes work together. For labor to happen, all the myocytes need to contract in synchrony, flexing in a wave-like manner across the whole uterus, to start working the baby down towards the exit. The transition requires each independent, and sometimes twitchy, myocyte to communicate and work together towards one goal: get baby out.

But if each myocyte is independent, how do they get their message straight?


From sporadic and independent to a coordinated cheering section – uterine contractions as a sports analogy

Ok, let me step back and emphasize again the insanity of the transformation that leads to the synchronized uterine contractions required to pop out a baby.

Since we’re dealing with a bunch of independent myocytes, I’m going to leave the superhero comparison behind and jump into an sports analogy. Bear with me, I’m not much of a sports analogy kind of gal…

In a matter of days, the uterus and its bundle of free-thinking myocytes essentially transform from a crowd watching a golfer sink a putt on the 18th hole to a rowdy football crowd in the fourth quarter of a playoff game. An audience that once solely focused on staying polite and quiet during tense moments turns into a crowd riled up en masse.


In a matter of days, the uterus and its bundle of free-thinking myocytes essentially transform from a crowd watching a golfer sink a putt on the 18th hole to a rowdy football crowd in the fourth quarter of a playoff game.


Like other smooth muscles (intestines, stomach, etc.), the uterus does not require the brain to tell it what to do, unlike our skeletal muscle. Flex your bicep. Good job. Now flex your uterus. Yup, you can’t do that.

The closest comparison might be to the heart and cardiac muscle cells[2].

The heart does not need a signal from the brain to get it’s independent cardiac muscle cells to contract in synchrony. Instead the heart has pacemaker cells that set the rhythm (like a conductor leading an orchestra).

The uterus has no centralized pacemaker.

Any one clump of myocytes can launch the wave that gets labor contractions in motion [3]. Similar to any individual sports fan starting an over-enthusiastic wave in the bleachers—going round and round until the final winning touchdown.

How does one random clump of myocytes start a wave of contractions that get bigger and stronger until every other myocyte works in sync towards baby expulsion? And how does the uterine transformation happen so quickly?


How does the uterine transformation happen so quickly?


Short answer — We don’t really know [2].

Long answer : there are a  lot of changes going on in those little cells in the weeks/days/hours leading up to labor. Some of which scientists are only beginning to understand.

What we do know (with help from my terrible sports analogy):

  1. The twitchy become twitchier. Myocytes increase excitability as they prepare for labor [4]. The muscular equivalent of passing around tequila shots to the golf crowd before magically transporting them to a playoff football game.
  2. The request to act becomes more persistent. The electrical signal, which causes each muscle cell to contract, lingers a bit longer. In our analogy – it’s shaming the guy on the left to get off his butt and contract/wave [5].
  3. The pressure is on. Increased internal pressure in the uterus encourages synchronization from myocytes on all sides of the uterus [6].  Raising the stakes and sending the game into extra innings (oh, and the fact that somehow golf just magically turned into a playoff football game.)
  4. Everyone links up. Little communication tunnels called gap junctions form between myocytes at the end of pregnancy. These small portals make neighboring myocytes respond and contract more quickly [4]. In our analogy: tying everyone’s hands together to make the wave involuntary and, therefore, spread around the stadium at an impressive pace.

All of these factors feed into each other in a loop, a positive feedback loop, a crescendo that increases how readily the uterine muscle cells contracts and how synchronized and strong the contractions are [4]. Hormone and hormones receptor changes at the end of pregnancy and once labor begins also fuel uterine contractions. With more and more synchronous contractions, pressure within the uterus (called intrauterine pressure, IUP) increases. These increases in pressure speeds along cervical dilation[7].

Open cervix, intense contractions, a bit of help from mom and… baby arrives into the world! With the placenta following close behind, of course.

Good job, uterus.

One of the puzzles of human pregnancy is the mechanism that generates the onset of labor.”

 — Smith et al. 2015


Want to help us solve a puzzle?

Bloomlife is the first wearable sensor that detects and measures uterine contractions. This glimpse into pregnancy physiology has never been seen before and the possibilities for what this might elucidate are very exciting. Already, within our first two years, we were able to capture data (thanks to our awesome Bloomlife moms!) that enabled us to find specific biomarker for detecting labor onset. In addition,our clinical research projects are expanding this work to further investigate the physiological changes that may signal labor and preterm labor.


[1] Young RC. Myocytes, myometrium, and uterine contractions. Ann N Y Acad Sci. 2007;1101: 72–84.

[2] Smith R, Imtiaz M, Banney D, Paul JW, Young RC. Why the heart is like an orchestra and the uterus is like a soccer crowd. Am J Obstet Gynecol. 2015;213: 181–185.

[3] Young RC. The uterine pacemaker of labor. Best Pract Res Clin Obstet Gynaecol. 2018; doi:10.1016/j.bpobgyn.2018.04.002

[4] Wray S, Burdyga T, Noble D, Noble K, Borysova L, Arrowsmith S. Progress in understanding electro-mechanical signalling in the myometrium. Acta Physiol . 2015;213: 417–431.

[5] Tong W-C, Tribe RM, Smith R, Taggart MJ. Computational modeling reveals key contributions of KCNQ and hERG currents to the malleability of uterine action potentials underpinning labor. PLoS One. 2014;9: e114034.

[6] Lammers WJEP. The electrical activities of the uterus during pregnancy. Reprod Sci. 2013;20: 182–189.

[7] Smith R. Parturition. N Engl J Med. 2007;356: 271–283.

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Molly Dickens, PhD

About Molly

Molly has her PhD in Physiology and spent over a decade as an academic research scientist slightly obsessed with the colliding worlds of brain science, hormones, stress and the reproductive system. Nowadays she heads up Content and Community at Bloomlife and edits Preg U. Science is still her jam and she can't help but continue to dive into the research world to find interesting bits about pregnancy and parenting.


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