Labor begins when a cascade of signals from both the baby and the mother’s body converge to shift the uterus from a quiet, pregnancy-maintaining state into an actively contracting one. There isn’t a single “start button.” Instead, hormonal shifts, physical stretching, inflammatory signals, and even the baby’s own lung development work together over days and weeks to trigger the process, typically between 39 and 41 weeks of pregnancy.
The Baby Sends the First Signal
One of the most striking discoveries about labor is that the baby plays an active role in starting it. As the fetal lungs mature in late pregnancy, they begin producing a substance called surfactant, which the baby will need to breathe air after birth. A key protein in that surfactant enters the amniotic fluid and sets off a chain reaction. It activates immune cells floating in the amniotic fluid, which then migrate to the wall of the uterus and trigger an inflammatory response that promotes contractions.
In mouse studies, injecting this protein directly into the amniotic sac caused premature delivery within one to two days, and blocking it with an antibody delayed labor by about 24 hours. Alongside the protein, the maturing lungs also release a potent inflammatory fat molecule into the amniotic fluid that further stimulates the uterine muscle to contract. The takeaway: labor is partly timed to when the baby’s lungs are ready. This is one reason babies born before 37 weeks often have breathing difficulties, as the signaling cascade hadn’t fully run its course.
Hormones Shift the Balance
Throughout pregnancy, progesterone keeps the uterine muscle relaxed and prevents contractions. In the days before labor, the balance between progesterone and estrogen changes at the level of the uterus itself. Women in active labor have significantly lower ratios of progesterone to estrogen in their amniotic fluid compared to women not yet in labor. This local hormonal shift doesn’t necessarily show up in a blood test, but it fundamentally changes how the uterine muscle behaves, making it more excitable and responsive to contraction signals.
Rising estrogen also increases the number of oxytocin receptors on uterine muscle cells. Oxytocin is the hormone most people associate with labor contractions, and for good reason, but the uterus can’t respond to it effectively until enough receptors are in place. Receptor levels peak right at the onset of labor, driven by the combination of hormonal changes, physical stretching of the uterus, and inflammation. Once the receptors are dense enough, even small amounts of oxytocin can trigger strong, coordinated contractions.
Physical Stretching Primes the Uterus
The sheer size of a full-term baby mechanically stretches the uterine wall, and that stretch is more than passive pressure. Uterine muscle cells contain specialized stretch-sensing channels that convert physical force into chemical signals. When these channels detect stretching, they allow calcium to flow into the muscle cells, which activates genes involved in contraction. One critical gene they switch on produces a gap junction protein called connexin 43. Gap junctions are tiny bridges between muscle cells that allow electrical signals to pass from one cell to the next, so the entire uterus can contract in a coordinated wave rather than in random, ineffective twitches.
This explains why labor tends to start earlier in twin or triplet pregnancies: greater uterine stretch triggers the pathway sooner. One set of these stretch channels also appears to play a role in the Ferguson reflex, where pressure from the baby’s head on the cervix triggers the brain to release more oxytocin, creating a positive feedback loop that intensifies contractions as labor progresses.
Inflammation Acts as a Final Trigger
Labor resembles an inflammatory event. In the days and hours before contractions begin, levels of inflammatory signaling molecules rise sharply in the amniotic fluid. These include several of the same molecules your body produces during an immune response. They serve a specific purpose here: they stimulate the local production of prostaglandins, hormone-like compounds that soften and thin the cervix (a process called ripening) while also enhancing uterine contractions.
This inflammatory wave comes from multiple sources at once. The baby’s lung surfactant activates immune cells. The shifting hormone ratios make the uterine lining more responsive to inflammatory signals. And the physical stretching of the uterus itself promotes inflammation. All of these converge into a self-reinforcing cycle: inflammation promotes prostaglandin production, prostaglandins promote more contractions, and contractions promote more inflammation, until labor becomes irreversible.
Why Labor Often Starts at Night
If you’ve heard that labor tends to begin in the middle of the night, there’s a biological reason. The onset of labor peaks between midnight and 5 a.m., regardless of gestational age. This timing traces back to melatonin, the hormone your brain releases after dark to regulate your sleep cycle. Melatonin binds to receptors on uterine muscle cells and amplifies the effect of oxytocin. In lab studies, combining melatonin with oxytocin doubled the contractile strength of uterine muscle cells compared to oxytocin alone.
Melatonin also boosts connexin 43, the same gap junction protein activated by uterine stretching. So at night, when melatonin levels are naturally high, the uterus is more sensitive to oxytocin and better able to coordinate contractions across its entire surface. This likely explains why early labor contractions often feel stronger at night and may seem to stall during the day.
What Full Term Actually Means
The medical definition of “term” is more specific than many people realize. A pregnancy is now classified as early term at 37 weeks through 38 weeks and 6 days, full term at 39 weeks through 40 weeks and 6 days, late term at 41 weeks through 41 weeks and 6 days, and post-term at 42 weeks and beyond. The earlier classification lumped everything from 37 to 42 weeks into a single “term” category, but outcomes are measurably better for babies born at 39 weeks or later, which is when the full signaling cascade from the baby’s lungs has typically completed.
Braxton Hicks vs. True Labor Contractions
In the weeks before labor, you’ll likely feel practice contractions known as Braxton Hicks. These can feel like a tightening across your belly, but they differ from real labor in several important ways:
- Pattern: Braxton Hicks are irregular and don’t get closer together. True labor contractions come at consistent, shortening intervals.
- Duration: Practice contractions vary in length. Real contractions last 30 to 90 seconds each.
- Intensity: Braxton Hicks are uncomfortable but manageable. You can walk and talk through them. True labor contractions grow progressively stronger, and talking or walking becomes difficult.
- Response to movement: Changing position or walking often stops Braxton Hicks. True contractions persist or intensify regardless of what you do.
- Location: Braxton Hicks are usually felt in the front of the belly. Real labor pain may radiate through the cervix, lower back, and entire abdomen.
True labor also comes with other signs that Braxton Hicks don’t produce, such as losing the mucus plug, a bloody show (pink or blood-tinged discharge), or your water breaking. If contractions are consistently getting stronger, longer, and closer together, and they don’t stop when you change positions, labor has likely begun.