Labor starts when a cascade of signals between your baby and your body reaches a tipping point. There’s no single switch that flips. Instead, hormonal shifts, physical changes in your uterus, and even a chemical signal from your baby’s lungs work together over days or weeks to move your body from pregnancy into active labor.
Your Baby Sends the First Signal
One of the most surprising discoveries about labor is that your baby plays an active role in starting it. As fetal lungs mature near the end of pregnancy, they begin secreting a protein into the amniotic fluid. This protein activates immune cells floating in that fluid, which then migrate to the wall of your uterus. Once there, they trigger inflammation and increase the production of compounds that make your uterine muscle contract more forcefully.
In animal studies, injecting this lung protein into amniotic fluid caused preterm delivery within 6 to 24 hours. Blocking it delayed labor by more than a day. The protein also appears to suppress the activity of progesterone, the hormone that has been keeping your uterus quiet for months. So in a very real sense, your baby’s body signals when it’s ready to be born, and your body responds.
The Progesterone-to-Estrogen Shift
Throughout pregnancy, progesterone keeps your uterine muscles relaxed and prevents contractions. As your due date approaches, the balance between progesterone and estrogen shifts. Estrogen levels climb relative to progesterone, and this changing ratio is one of the central drivers of labor onset.
In most mammals, this shift happens because progesterone levels physically drop in the bloodstream. Human pregnancy works differently. Progesterone levels don’t actually fall before labor begins. Instead, your body becomes less responsive to progesterone at the level of the uterine muscle itself, a process sometimes called “functional progesterone withdrawal.” The inflammatory signals from your baby’s lung protein contribute to this by interfering with how progesterone binds and works inside uterine cells. The result is the same: your uterus loses its progesterone-driven calm and becomes increasingly ready to contract.
How Your Uterus Becomes Primed to Contract
The rising estrogen-to-progesterone ratio triggers a dramatic change in your uterine muscle. The number of oxytocin receptors on uterine muscle cells increases up to 200-fold by the end of pregnancy. This is why oxytocin, often called the “labor hormone,” has relatively little effect on your uterus early in pregnancy but becomes powerfully effective near term. It’s not that your body suddenly produces huge amounts of oxytocin. Your uterus simply becomes far more sensitive to the oxytocin already circulating.
When oxytocin binds to these receptors, it does two things. First, it directly triggers muscle contractions by opening calcium channels inside smooth muscle cells, which is the basic mechanism behind any muscle contraction. Second, it stimulates your body to produce prostaglandins, which are hormone-like compounds that do their own important work. Prostaglandins help form connections (called gap junctions) between individual muscle cells, allowing them to contract in coordinated waves rather than randomly. They also further increase the number of oxytocin receptors, creating a feedback loop that intensifies contractions over time.
How Your Cervix Prepares
While your uterus is becoming more contractile, your cervix is undergoing its own transformation. For most of pregnancy, the cervix is firm, thick, and tightly closed. In the weeks before labor, prostaglandins and other inflammatory compounds soften and thin it in a process called cervical ripening. Immune cells, particularly macrophages, accumulate in the cervix and release prostaglandins and other signals that break down the dense connective tissue holding it closed.
This is why your care provider checks your cervix in late pregnancy. Softening and early dilation can begin days or even weeks before contractions start, and it’s a sign that the biochemical groundwork for labor is underway. Without adequate cervical ripening, even strong contractions won’t lead to effective labor, which is one reason inductions sometimes use prostaglandin-based medications before starting oxytocin.
Early Signs That Labor Is Approaching
Before contractions begin in earnest, your body often gives signals that labor is getting close. One of the most talked-about is the “bloody show,” a small amount of blood-tinged mucus that passes from your cervix as it begins to thin and open. For some people this happens weeks before labor. For others it happens during labor itself. The timing varies so widely that it’s better understood as a sign that your cervix is changing rather than a reliable countdown to delivery.
Your water breaking before contractions start happens in roughly 10% of pregnancies. Most of the time, contractions are already underway when the amniotic sac ruptures. If your water does break first, labor typically follows within hours, though the timeline varies.
Telling Real Contractions From Practice Ones
Braxton Hicks contractions can start in the second trimester and become more noticeable as your due date approaches. They’re irregular, usually painless or mildly uncomfortable, and they stop when you change position or activity. True labor contractions are different in three specific ways.
- Regularity. True contractions settle into a predictable pattern, coming at roughly even intervals that gradually shorten over time.
- Duration. Each contraction lasts longer as labor progresses, rather than staying the same length or fading.
- Intensity. They get stronger, not weaker, and don’t ease up when you move or rest.
The general guideline is to contact your care provider or hospital when contractions are coming every five minutes and have held that pattern for at least one hour. By that point, they’re typically strong enough that you need to pause and breathe through them.
When Labor Typically Starts
Full term runs from 37 to 42 weeks, and there’s a wide window of normal. Most spontaneous labor happens between 39 and 41 weeks, but predicting exactly when is notoriously difficult. Among pregnancies that reach 41 weeks without complications, about 68% will go into spontaneous labor before 42 weeks. The rest may need medical induction.
First-time mothers tend to go into labor later than those who’ve given birth before. Factors like genetics, the length of your own mother’s pregnancies, and your baby’s size and position all play a role. But ultimately, labor starts when the combined signals from your baby, your hormones, your immune system, and your uterus converge. It’s a process that builds over weeks, even if it feels like it happens all at once.