Labor starts when a cascade of signals between your baby and your body reaches a tipping point, triggering hormonal shifts, physical changes in your cervix, and coordinated uterine contractions. No single switch flips to start the process. Instead, several biological systems build toward labor over days or weeks, and the baby itself plays a surprisingly active role in deciding when it’s time.
Your Baby Sends the First Signal
One of the most remarkable discoveries about labor is that the baby’s lungs help trigger it. As the fetal lungs mature near the end of pregnancy, they begin secreting a protein called surfactant protein A into the amniotic fluid. This protein activates immune cells floating in the amniotic fluid, which then migrate to the wall of the uterus. Once there, they set off an inflammatory signaling cascade that primes the uterus for contractions. Research published in the Proceedings of the National Academy of Sciences described this surfactant protein as essentially a “hormone of parturition,” a chemical message from the baby saying its lungs are developed enough to breathe air. In other words, your baby’s body helps decide when it’s ready to be born.
How Hormones Ramp Up Contractions
Oxytocin is the hormone most people associate with labor, and for good reason. Produced in the brain and released from the pituitary gland, oxytocin binds to receptors on the muscle cells of the uterus and causes them to contract. During labor, it’s released in pulses that grow stronger and more frequent as labor progresses. There’s also a feedback loop at work: as the baby’s head presses down on the cervix, nerve signals travel to the brain and trigger more oxytocin release. This is called the Ferguson reflex, and it’s why contractions tend to intensify as the baby descends.
Oxytocin also stimulates the production of prostaglandins, hormone-like compounds that soften and thin the cervix while also boosting uterine contractions. These two systems reinforce each other, creating the escalating pattern of labor.
Your Cervix Undergoes a Dramatic Transformation
Before contractions can push a baby out, the cervix has to change from a firm, closed structure into something soft and stretchy enough to open to 10 centimeters. This process, called cervical ripening, is driven largely by inflammation. In the final days of pregnancy, immune cells flood into the cervical tissue: neutrophils, macrophages, and mast cells. These cells release enzymes that break down collagen fibers, the structural protein that keeps the cervix rigid. By the time labor begins, collagen concentration in the cervix drops by roughly 70%. Fluid also increases in the tissue, making it pliable.
This ripening process explains why your healthcare provider checks your cervix in late pregnancy. They’re looking for signs that this remodeling is underway, using measurements like dilation, thinning (effacement), and the baby’s position. These factors are scored on something called the Bishop score. A higher score means your cervix is more “favorable,” or closer to being ready for labor. Generally, a score above 8 suggests labor could start on its own or be successfully induced.
How Your Uterus Learns to Contract Together
Your uterus is a muscle, but for most of pregnancy its cells contract independently, producing only weak, uncoordinated tightenings (Braxton Hicks contractions). What changes near labor is the formation of tiny channels between muscle cells called gap junctions. These channels allow electrical signals and chemical messengers to pass directly from one cell to the next, so that when one cell contracts, its neighbors contract in sync. Without these gap junctions, labor contractions cannot occur. Their assembly is one of the final steps in preparing the uterus, and it’s regulated in part by progesterone activity shifting in the weeks before delivery.
Early Signs That Labor Is Approaching
Several physical changes signal that your body is moving toward labor, even before regular contractions begin. Losing your mucus plug, a thick clump of mucus that sealed the cervix throughout pregnancy, is one of the most recognized signs. It can appear as a glob of clear, pink, or slightly bloody discharge. Sometimes it comes out all at once, sometimes gradually. The timing after losing it varies widely: you might go into labor within hours, or it could still be weeks away. It typically comes out during early labor, when the cervix has dilated to a few centimeters.
Bloody show, a small amount of blood-tinged mucus caused by tiny blood vessels breaking as the cervix opens, is a related but slightly different sign. It tends to appear closer to active labor than the mucus plug alone. Other common prelabor signs include the baby dropping lower into the pelvis, increased pelvic pressure, more frequent Braxton Hicks contractions, and loose stools as prostaglandins affect the bowel.
What Actually Works to Start Labor Naturally
Many people near their due date want to know if there’s anything they can do to get things moving. The evidence is mixed for most popular methods, but a few have real data behind them.
Nipple stimulation is the most studied natural approach. A Cochrane review found that women who used breast stimulation were significantly more likely to be in labor within 72 hours compared to those who did nothing: about 37% of the stimulation group went into labor versus only 6% of the control group. It works by triggering oxytocin release. No cases of dangerous over-stimulation of the uterus were reported in the trials, and women who used it also had lower rates of postpartum hemorrhage.
Walking in late pregnancy has been shown in clinical trials to improve cervical readiness. Women who walked regularly had higher Bishop scores at admission, were more likely to go into spontaneous labor, and were less likely to need induction or cesarean delivery. The mechanism likely involves gravity helping the baby’s head press on the cervix (triggering the Ferguson reflex) along with hormonal and metabolic changes from physical activity. “Curb walking,” where you walk with one foot on a curb to create an uneven stride, is a popular variation, though studies have tested regular walking rather than this specific technique.
Castor oil has been used for centuries but the evidence is less encouraging. In a large study of over 1,000 women, about half who took castor oil went into labor without needing any additional intervention. However, those who used castor oil actually had a longer average time from induction to delivery (about 27 hours versus 19 hours with standard methods), and roughly half still needed additional medical induction. Castor oil works by being converted into a compound that binds to prostaglandin receptors in smooth muscle, but it also affects the intestines, commonly causing diarrhea and nausea.
How Medical Induction Works
When labor needs to be started for medical reasons, the approach depends on how ready your cervix is. If your Bishop score is low, meaning the cervix is still firm and closed, the first step is usually cervical ripening. This can involve prostaglandin medications placed near the cervix to mimic the natural softening process, or a small balloon catheter inserted through the cervix to apply gentle mechanical pressure.
Once the cervix is favorable, synthetic oxytocin delivered through an IV is the most common way to start and strengthen contractions. It mimics what your body does naturally, but allows the medical team to control the dose. In some cases, breaking the amniotic sac (amniotomy) is used alongside these methods to speed things up.
The success of induction depends heavily on where your body already is in the process. A cervix that has already started softening and opening responds much more quickly than one that hasn’t begun to change. This is why some inductions take just a few hours while others stretch over a day or more.