Another Mystery of How Childbirth Works Discovered

As we learn more about the
human body, its complexity
becomes increasingly apparent

Another Mystery of How Childbirth Works Discovered

by Jerry Bergman, PhD

As we learn more about the human body, its complexity becomes increasingly apparent. During childbirth, the primary force that delivers the baby is the rhythmic tightening and relaxing of the uterine muscles. The uterus contains a thick, muscular middle layer called the myometrium, which is composed of interwoven bundles of smooth muscle supported by connective tissues and ligaments. During labor, these muscle bundles contract powerfully, generating coordinated forces that both push the baby through the birth canal and cause the cervix to dilate and efface.

Cervical effacement refers to the thinning and shortening of the cervix in preparation for delivery and is measured from 0 percent (thick) to 100 percent (fully thinned). Together, dilation and effacement allow the baby to pass through the cervix and exit the mother’s body.

Hormones and Mechanics

In the college-level Anatomy and Physiology course, I taught that uterine contractions occur primarily in response to hormonal signaling. Oxytocin, in particular, is described as stimulating the powerful contractions of the uterine muscle during labor, with prostaglandins acting to amplify this effect. Textbooks commonly define uterine contraction as “the rhythmic tightening and relaxation of the uterine myometrium, which increases in strength and frequency during labor to facilitate the expulsion of the fetus,” noting that these contractions are regulated by hormones such as oxytocin and influenced by cervical stretching.

However, this explanation—taught by myself and thousands of anatomy professors—was not so much incorrect as incomplete. Until recently, anatomists were unaware of a major functional component of the uterus, and as a result, no additional mechanism contributing to the delivery of the baby was described in our textbooks.

Now research has now documented that “childbirth isn’t just chemical—it’s mechanical, with the uterus using pressure sensors to keep labor on rhythm.”[1] The Scripps Research Institute explained the implications of this discovery:

Childbirth depends not just on hormones, but on the uterus’s ability to sense physical force. Scientists found that pressure and stretch sensors in uterine muscles and surrounding nerves work together to trigger coordinated contractions. When these sensors are disrupted, contractions weaken, and delivery slows. The discovery helps explain stalled labor—and could one day lead to better ways to manage childbirth.[2]

Specifically, the molecular mechanisms by which the uterus detects and responds to these physical forces remain an active area of research. A better understanding of these processes is essential for explaining why labor sometimes begins prematurely or, conversely, fails to progress, placing both the mother and the baby at risk.

Basic parts of the uterus. From Wikimedia Commons.

Irreducible Complexity of the Uterus

The researchers are only beginning to understand the irreducible complexity of uterine function during pregnancy and childbirth. Thanks to this research, a promising new door has been opened that will help us better understand this complex system. Zhang et al. admit that: “Mechanical forces play key roles at various stages of pregnancy and parturition, but the detailed mechanisms underlying these events are not yet well understood.”[3]

It is known that, as the fetus grows, the uterus undergoes dramatic expansion, adapting structurally and functionally to accommodate fetal development. This stretching activates mechanosensitive signaling pathways that help coordinate uterine muscle remodeling and, later, guide the synchronized contractions required for labor. Precise regulation of the uterine expansion and contraction is critical: inadequate uterine responsiveness can delay or arrest labor, while excessive or abnormal mechanical stress can contribute to fetal distress and maternal complications, possibly even squeezing the baby to death.

Piezoelectricity is the ability of certain materials called Piezo materials, such as crystals, ceramics, and even bone, to generate an electric charge when subjected to mechanical stress, such as squeezing and pressing. In contrast, mechanotransduction is the process by which cells convert mechanical forces into electrical and biochemical signals. In the uterus, this process is mediated in part by mechanosensitive ion channels formed by the proteins PIEZO1 and PIEZO2. These channels enable uterine and sensory cells to detect stretch and pressure and to respond in a coordinated manner during labor. Research by Yunxiao et al. demonstrated that PIEZO1 and PIEZO2 play distinct but complementary roles in parturition.

How Mechanosensitive Ion Channels Work

PIEZO1 is primarily expressed in uterine smooth muscle, where it responds to increasing mechanical tension as contractions intensify. In contrast, PIEZO2 is localized mainly in sensory neurons innervating the cervix and vagina, where it is activated as fetal descent stretches these tissues. Activation of PIEZO2 triggers a neural reflex that further enhances uterine contractions. Together, these mechanosensitive pathways convert physical forces into synchronized electrical and chemical signals that help coordinate effective labor. Importantly, the system exhibits functional redundancy—overdesign: disruption of one pathway can be partially compensated by the other, allowing labor to proceed under a range of physiological conditions.

As far as is known, this system is very similar in all mammals that give live birth. Even this brief outline illustrates the enormous and substantial functional complexity of the uterine system, which integrates mechanical sensing, neural signaling, and coordinated muscle contraction. How women successfully delivered their babies before these systems evolved was never discussed in the papers I consulted about the Zhang et al research. None of the articles I reviewed mentioned evolution, but this remarkably complex system of childbirth, including PIEZO1 and PIEZO2 receptors are assumed to have arisen through gradual modification of earlier mechanosensory functions.

The problem for evolution is, both the hormone control system and the mechanosensitive pathways must be established before normal successful delivery can be achieved. Think of how many mothers and babies must have died due to delivery difficulties until the system evolved!

The researchers themselves acknowledge that “mechanical forces are extensively involved in pregnancy and parturition, but their precise roles and mechanisms remain poorly understood.” As additional details of these pathways are uncovered, the system appears increasingly intricate, raising important questions about how such tightly coordinated mechanisms could have originated, regulated, and maintained by evolution.

Summary

This review adds to the growing body of evidence demonstrating that childbirth depends not only on hormonal cues but also on mechanosensitive systems that allow the uterus to respond appropriately to physical forces during labor. Such findings raise broader questions about how highly coordinated biological systems like this could have evolved and how they are maintained. This review is only one more example of this trend.

Now we know that childbirth is caused, not only by hormones, but also because the uterus “knows” when to push during childbirth. Each new evaluation of human physiology reveals increasing levels of functional complexity and integration. This review adds to that growing body of evidence by demonstrating that childbirth depends not only on hormonal cues but also on mechanosensitive systems that allow the uterus to respond appropriately to physical forces during labor.

Such findings raise broader questions about how highly coordinated biological systems could have evolved and how delivery could have normally occurred until they evolved.

References

[1] Scripps Research Institute. 2026. Scientists discover how the uterus knows when to push during childbirth. Childbirth isn’t just chemical—it’s mechanical, with the uterus using pressure sensors to keep labor on rhythm; https://www.sciencedaily.com/releases/2026/01/260112214313.htm., January 13.

[2] Scripps Research Institute, 2026.

[3] Zhang, Yunxiao, et al. 2025. PIEZO channels link mechanical forces to uterine contractions in parturition. Science 390(6774), DOI: 10.1126/science.ady3045.

Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,900 publications in 14 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,800 college libraries in 27 countries. So far over 80,000 copies of the 60 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.