These are the top theories about what causes labor to start.
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3 Top Theories About What Causes Labor To Start
by Jill Di Donato
Originally Published: 

Although there are plenty of hypotheses regarding labor and delivery, no one really knows what triggers labor. It's one of life's mysteries, I suppose, and a pretty vital one at that. That being said, there are top theories about what causes labor to start.

Sure, it's nice to be surprised by water breaking, but wouldn't it be nice to know what causes you to go from pregnant to parent in a matter of minutes (OK, more like hours)? Unfortunately, there’s really not one singular trigger responsible for starting the process, as Taraneh Shirazian M.D., OB/GYN at NYU and founder of Mommy Matters tells Romper.

“Immunoproteins get released that start ripening the cervix, that little bit of dilation triggers contractions, then those contractions trigger a whole cascade of events,” she explains.

As the website Midwives Online reported, the medical community traditionally believed that the mother's body caused labor to start. However, there's been a shift in thinking more recently which suggests it's the baby sending those signals to the mother's body. In other words, think of the baby acting like a timer letting you know the bun in the oven is ready.

Even though each pregnancy and birth is different, there are always three stages of labor, noted Baby Center. Labor begins with the dilation of the cervix as it begins to open. This is called early labor. You then move into active labor, when the cervix is fully dilated and contractions come frequently. Finally, the third stage of labor begins when you start to push, and you deliver your baby and placenta. That's all pretty straightforward. But as for what causes labor to start in the first place, read on to discover some of the top theories.


Mother’s Pituitary Gland Secretes Oxytocin

One theory about what triggers labor is that the mother's body, specifically her pituitary gland, begins to secrete oxytocin when the baby is fully developed and ready to be born, according to Medical News Today. Oxytocin is the hormone that stimulates contractions, reading the cervix for labor. The more mobile the cervix gets, the more the vagina widens, and labor ensues.


The Baby's Lungs Signal Labor

In order to achieve a vaginal birth, a laboring person's cervix has to reach 10 cm dilation. Shutterstock

The theory that the baby signals labor is a shift away from previous theories that postulated that the mother's body triggers labor.

Researchers Carole Mendelson, Jennifer Condon and Pancharatnam Jeyasuria published a study in the Proceedings of the National Academy of Sciences that indicated fetal lungs provide the signal to initiate labor. When the fetal lungs have reached full maturation, they produce a certain type of protein, which in turn sends a wave of prostaglandins through the mother's body that trigger labor. According to Mayo Clinic, the baby's lungs are the last organ to develop in utero.

"We found that a protein within lung surfactant serves as a hormone of labor that signals to the mother's uterus when the fetal lungs are sufficiently mature to withstand the critical transition to air breathing," Dr. Mendelson told SheKnows.

"No one really understands what causes normal or preterm labor. There may be several chemical pathways that lead to labor, but we think that this surfactant protein, which is also produced by the fetal lung in humans, may be the first hormonal signal for labor," she said.


The Baby's Adrenal Glands Signal Labor

As Baby Center explained, doctors theorize that maturation of your baby's adrenal glands might trigger labor. Although your baby's brain is fully developed in the second trimester, rapid brain development occurs around week 25 to 28. However, it's not until weeks 38 to 40 that your baby's adrenal glands are engaged in what's called the HPA Axis (the interdependent relationship between the hypothalamus, pituitary gland, and adrenal glands). The fetal adrenal gland functions like an adult adrenal gland in many ways, according to the American Academy of Pediatrics.

One theory about what triggers labor is that when your baby's brain functions are on fleek, and that includes having a fully developed HPA Axis, the fetal adrenal glands secrete the hormone cortisol. The release of cortisol in utero causes a hormonal disturbance of progesterone and estrogen, and labor begins.


Condon, J., Jeyasuria, P., Faust, J. and Mendelson, C., (2004), "Surfactant protein secreted by the maturing mouse fetal lung acts as a hormone that signals the initiation of parturition," Proceedings of the National Academy of Sciences of the United States of America,


Taraneh Shirazian M.D., OB/GYN at NYU and founder of Mommy Matters

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