Your body doesn’t flip a single switch to start labor. Instead, a cascade of signals builds over weeks, originating from the baby, the placenta, and your uterus, all converging until contractions begin and your cervix opens. The timing depends on a biological countdown that researchers call the “placental clock,” combined with signals from your baby’s maturing lungs and a dramatic shift in how your body responds to its own hormones.
The Placental Clock Sets the Timeline
The placenta produces a stress hormone called CRH that rises exponentially throughout pregnancy. This hormone acts as a kind of countdown timer. A landmark 1995 study found that CRH levels in the mother’s blood could predict delivery timing surprisingly early: women who delivered preterm had higher CRH levels as early as 16 to 18 weeks, while women who went past their due date had lower levels at the same point. The differences weren’t subtle. They suggested that the pace of CRH production is set relatively early in pregnancy and determines when the cascade toward labor reaches critical mass.
For most of pregnancy, a binding protein in your blood neutralizes CRH, keeping it inactive. But as CRH production accelerates in the final weeks, it overwhelms that binding protein. The now-free CRH does several things at once: it boosts the production of prostaglandins in the uterine membranes (which soften the cervix and trigger contractions), it makes the uterus more responsive to oxytocin, and it stimulates estrogen production. Each of these effects, in turn, drives even more CRH release, creating a series of positive feedback loops that amplify progressively until labor begins.
Your Baby’s Lungs Send the Signal
One of the most remarkable discoveries about labor is that the baby plays an active role in starting it. As the fetal lungs mature, they begin secreting a protein called surfactant protein A (SP-A) into the amniotic fluid. In mice, this protein first appears around the time the lungs are nearly ready for breathing and rises steadily toward the due date.
SP-A activates immune cells floating in the amniotic fluid, causing them to migrate to the wall of the uterus. Once there, these cells trigger inflammatory signaling that increases uterine contractility. In effect, the baby’s lungs send a chemical message saying “I’m ready to breathe,” and the mother’s body responds by initiating labor. This finding, published in the Proceedings of the National Academy of Sciences, was one of the first to demonstrate a direct link between fetal organ maturity and the start of birth.
Progesterone Loses Its Grip
Progesterone is the hormone that keeps your uterus quiet for nine months. It suppresses contractions, blocks the formation of gap junctions between muscle cells (which need to connect for coordinated contracting), and prevents the cervix from softening prematurely. In many mammals, progesterone levels simply drop before labor. In humans, though, blood levels of progesterone don’t fall. Instead, the body pulls off something more subtle.
In the weeks before labor, the uterine muscle changes which type of progesterone receptor it produces. It shifts toward a version that actually suppresses progesterone’s calming effects. The result is a “functional progesterone withdrawal,” where progesterone is still circulating but the uterus stops listening to it. Once that happens, the uterus becomes responsive to stretch, to oxytocin, and to prostaglandins. It’s like removing the brakes from a car that’s already pointed downhill.
The Uterus Becomes Primed to Contract
With progesterone’s influence fading, two other changes prepare the uterine muscle for coordinated labor contractions. First, the number of oxytocin receptors in the uterine wall increases roughly twelvefold between mid-pregnancy and 37 to 41 weeks. After labor actually begins, receptor levels climb even higher. This explains why oxytocin, which circulates throughout pregnancy, only triggers powerful contractions near the end: the uterus literally couldn’t respond to it before. In cases where labor induction with synthetic oxytocin fails, receptor levels tend to be significantly lower than in women who go into labor spontaneously.
Second, the growing baby stretches the uterine wall, and that mechanical stretch activates genes for gap junction proteins and contractile proteins. These gap junctions are electrical bridges between muscle cells that allow them to contract in unison rather than in isolated, ineffective twitches. Progesterone normally blocks this process, but once the functional withdrawal occurs, stretch becomes a powerful activator.
Your Cervix Remodels Itself
The cervix is not just a passive doorway. For most of pregnancy it stays firm and closed, held together by densely packed collagen fibers with strong chemical cross-links. In the final days before labor, a process called cervical ripening breaks down that structure. Collagen concentration drops by nearly 70% at term. The tightly organized fibers become dispersed and randomly oriented, fluid content increases, and the tissue transforms from something rigid into something soft and stretchable.
Prostaglandins drive much of this remodeling. One type in particular, PGE2, stimulates enzymes that cleave the cross-links between collagen fibers and increases water content in the tissue. Inflammatory signaling molecules amplify the process, recruiting immune cells that further break down the cervical structure. This is why your cervix can go from completely closed to fully dilated over the course of hours once active labor is underway. The groundwork was laid days or even weeks earlier.
Braxton Hicks vs. the Real Thing
Many people feel contractions weeks before labor begins and wonder whether the process has started. Braxton Hicks contractions are irregular, unpredictable, and generally more uncomfortable than truly painful. They don’t follow a pattern, they don’t get closer together over time, and they often stop if you change position or activity. If you can sleep through a contraction, it’s a Braxton Hicks contraction. These contractions don’t dilate the cervix, though they may play a small role in softening it.
True labor contractions are fundamentally different. They come at regular intervals that shorten over time, last between 30 and 90 seconds, and grow stronger rather than fading. They continue regardless of movement or rest. On examination, true labor contractions produce measurable changes in cervical dilation and thinning. The distinction isn’t just about pain level. It’s about whether the coordinated hormonal and structural cascade described above has actually engaged.
How It All Comes Together
Labor isn’t triggered by a single event. It’s the result of multiple systems reaching a tipping point at roughly the same time. The placental clock drives CRH to levels that overwhelm its binding protein. The baby’s lungs signal readiness through surfactant proteins. The uterus stops responding to progesterone’s calming influence. Oxytocin receptors multiply. Gap junctions wire the uterine muscle for coordinated action. Prostaglandins soften and thin the cervix. Each process feeds into the others through positive feedback loops, so once the cascade begins, it accelerates.
The first stage of labor begins with regular contractions and ends when the cervix reaches 10 centimeters of dilation. Early labor (the latent phase) involves gradual, slower cervical change. The shift into active labor, where dilation speeds up considerably, was traditionally thought to occur around 4 centimeters. Contractions are the most common first symptom, reported far more often than water breaking as the initial sign. In a review of 91 studies, contractions and labor pain were mentioned as onset symptoms in about 86% of the literature, making them the most recognized signal that the cascade has reached its final stage.