Your body doesn’t flip a single switch to start labor. Instead, it follows a cascade of signals that begin with the baby itself and build over days or weeks, involving hormones from the placenta, physical stretching of the uterus, and a carefully timed withdrawal of the hormone that kept your uterus quiet for nine months. Here’s how each piece works together to tell your body it’s time.
The Baby Sends the First Signal
One of the most surprising discoveries about labor is that the baby’s lungs help start it. As fetal lungs mature in late pregnancy, they begin producing a protein called surfactant protein A (SP-A), the same substance that allows the lungs to inflate properly after birth. SP-A levels in the amniotic fluid rise sharply in the final days before delivery.
This protein activates immune cells floating in the amniotic fluid. Those activated cells then migrate from the fluid into the wall of the uterus, where they trigger an inflammatory response. That inflammation flips on genes that make the uterine muscle more contractile. In mouse studies, injecting SP-A directly into the amniotic sac caused premature delivery from that side of the uterus only, with immune cells arriving in the uterine wall within four and a half hours. The other side, which wasn’t injected, stayed quiet. This suggests the signal is remarkably local and specific: the baby’s lungs essentially tell the uterus “I’m ready to breathe.”
The Placental Clock
The placenta runs its own countdown. Starting in mid-pregnancy, it produces increasing amounts of corticotropin-releasing hormone (CRH), a stress hormone normally made in the brain. CRH levels in the mother’s blood rise exponentially as pregnancy progresses, and the shape of that curve appears to predict when delivery will happen. In women who deliver preterm, the curve accelerates earlier. In women who go past their due date, it rises more slowly.
This pattern led researchers to propose the idea of a “placental clock,” a biological timer set early in pregnancy that determines how long gestation will last. Placental CRH also helps mature the baby’s organs and influences blood flow between the placenta and the fetus, meaning it’s simultaneously preparing the baby for life outside the womb while counting down to delivery.
How Progesterone Loses Its Grip
Progesterone is the hormone that keeps the uterus relaxed throughout pregnancy. It actively suppresses the genes responsible for making the uterine muscle contract. For labor to begin, that suppression has to end. In many mammals, this happens through a straightforward drop in blood progesterone levels. Humans do something more complicated.
In human labor, blood levels of progesterone stay high. Instead, the uterine muscle itself breaks progesterone down. Cells in the uterine wall ramp up production of an enzyme that converts progesterone into an inactive form. This means that even though plenty of progesterone circulates in the blood, levels inside the uterine muscle cells drop. Without progesterone bound to its receptor, the receptor changes behavior dramatically: it stops suppressing contraction genes and paradoxically starts activating them. The result is a uterus that becomes increasingly responsive to contraction signals, all while blood tests would show “normal” progesterone levels.
This functional withdrawal of progesterone unlocks the expression of key proteins, including one that forms gap junctions between muscle cells, allowing them to contract in sync rather than independently. Without those gap junctions, contractions would be disorganized and ineffective.
Oxytocin Receptors Multiply
Oxytocin gets most of the credit for causing contractions, but the hormone itself isn’t what changes most before labor. What changes is how sensitive the uterus is to it. The number of oxytocin receptors in the uterine muscle increases up to 200-fold toward the end of pregnancy. This massive increase is driven by rising estrogen levels relative to progesterone. So even normal circulating levels of oxytocin, which have been present throughout pregnancy, suddenly become far more potent because the uterus is now equipped to respond to them.
Physical Stretch Plays a Role
The sheer size of the baby matters too. As the uterus stretches in late pregnancy, specialized pressure-sensing channels in the uterine muscle and in the nerves running through the birth canal convert that mechanical force into chemical signals. Research published in Science identified two types of these channels working in complementary ways: one type sits in the uterine muscle itself and responds to pre-labor contractions by turning on genes that make the muscle more contractile. The other type sits in sensory nerves in the birth canal and triggers what’s known as the Ferguson reflex, a feedback loop where pressure on the cervix signals the brain to release more oxytocin, which causes stronger contractions, which pushes the baby down further, which creates more pressure.
This is one reason why labor tends to accelerate once it’s underway, and why women carrying twins or larger babies sometimes deliver earlier. More stretch means earlier activation of these mechanical pathways.
Prostaglandins Soften the Cervix and Drive Contractions
Two types of prostaglandins handle separate but equally important jobs. One (PGE2) is primarily responsible for softening and thinning the cervix, breaking down the dense collagen that kept it sealed shut. The other (PGF2α) is a powerful stimulant of uterine contractions, producing consistent, sustained tightening of the muscle. PGE2 actually has a more complex effect on the uterus itself, causing an initial contraction followed by relaxation, which is why it’s more useful for cervical preparation than for driving labor forward.
The inflammatory response triggered by the baby’s lung signals increases production of both prostaglandins. PGF2α also circulates to the ovaries, where it contributes to reducing progesterone production, further tipping the balance toward labor.
Braxton Hicks vs. True Labor Contractions
All of these signals build gradually, which is why many women experience weeks of “practice” contractions before the real thing. Braxton Hicks contractions are irregular, usually felt only in the front of the abdomen or in one specific area, and they tend to stay the same intensity or fade away. True labor contractions follow a recognizable pattern: they start in the mid-back and wrap around to the front, they get progressively stronger, and they come at increasingly regular intervals. Braxton Hicks contractions do not dilate the cervix, no matter how uncomfortable they feel.
Other pre-labor signs include the loss of the mucus plug, which may appear as brownish or blood-tinged discharge. This “bloody show” means the cervix has started to change, but active labor can still be days away.
When All the Signals Converge
Labor isn’t defined by any single event. Clinically, it begins when regular, painful contractions produce progressive dilation or thinning of the cervix. The American College of Obstetricians and Gynecologists considers 6 centimeters of cervical dilation the start of active labor, the phase where things typically progress steadily toward delivery.
For practical purposes, the commonly used guideline is the 5-1-1 rule: head to the hospital when contractions come every 5 minutes, last at least 1 minute each, and have followed this pattern consistently for 1 hour. Early labor contractions may last only 30 to 45 seconds and come anywhere from 5 to 30 minutes apart, so there’s often a long ramp-up before that threshold is met.
What makes the whole system remarkable is its redundancy. The baby’s lungs, the placental clock, progesterone withdrawal, oxytocin receptor buildup, physical stretch, and prostaglandin production all feed into each other. No single pathway has to work perfectly because they reinforce one another. This is why labor, once it truly begins, tends to build its own momentum, and why the body rarely “misses” the signal entirely.