Labor starts naturally through a cascade of signals between your baby and your body, building over days to weeks before contractions become regular and strong. There isn’t a single switch that flips. Instead, hormonal shifts, physical changes in your cervix, and even a signal from your baby’s lungs work together to move your body from pregnancy into active labor. Understanding this process can help you recognize what’s happening in the weeks and days before delivery.
Your Baby Sends the First Signal
One of the most fascinating discoveries about labor is that your baby plays an active role in starting it. Research published in the Proceedings of the National Academy of Sciences found that as the fetal lungs mature near the end of pregnancy, they begin secreting a protein called surfactant protein A (SP-A) into the amniotic fluid. This protein activates immune cells in the amniotic fluid, which then migrate to the wall of the uterus and trigger an inflammatory response. That inflammation is one of the key forces that gets the uterus ready to contract.
In other words, when your baby’s lungs are developed enough to breathe air, they release a chemical signal telling your body it’s time. The researchers described SP-A as essentially a “hormone of parturition,” meaning it functions as a hormonal trigger for birth. This mechanism helps explain why labor typically starts when it does: the timing is linked to your baby’s readiness for life outside the womb.
The Hormonal Buildup Before Labor
While your baby is sending signals, your own hormones are shifting dramatically in the final weeks of pregnancy. Estrogen levels rise, activating the uterine muscle so it can contract efficiently. At the same time, the number of oxytocin receptors in your uterus increases significantly. Think of it like adding more locks that oxytocin can open: even before oxytocin levels spike, your uterus is becoming more and more sensitive to it.
Prostaglandins, hormone-like compounds produced locally in the uterus and cervix, also increase. They soften and thin your cervix (a process called ripening) so it can dilate when contractions begin. Rising inflammation in the uterus and cervix, though it sounds alarming, is a normal and necessary part of this preparation. All of these changes happen gradually, which is why the process can feel slow and ambiguous in the days leading up to active labor.
Once labor is underway, oxytocin takes center stage. It’s released from your brain into your bloodstream in pulses, driving rhythmic uterine contractions. As labor progresses, the pressure of your baby’s head against the cervix triggers even more oxytocin release, creating a positive feedback loop known as the Ferguson reflex. This is what powers the intense contractions of late labor and pushing. Prolactin, better known for its role in milk production, also stimulates additional oxytocin release during this stage.
How Your Uterus Prepares to Contract
Your uterus is a muscle, but for most of pregnancy it stays relatively quiet. One reason is that the individual muscle cells aren’t well connected to each other early on. Near the end of pregnancy, however, your body builds more gap junctions between the uterine muscle cells. These are tiny communication channels that allow the cells to contract together in a coordinated wave rather than firing randomly. The increase in these connections is one reason why late-pregnancy contractions feel fundamentally different from the mild tightening you may have noticed earlier.
Early Signs That Labor Is Approaching
Several physical changes signal that your body is getting ready, though none of them tell you exactly when labor will start.
The mucus plug, a thick collection of mucus that seals the cervix during pregnancy, may come out as your cervix begins to soften and open. You might notice it as a glob of clear, pink, or blood-tinged mucus. Some people go into labor within hours of losing it. Others don’t deliver for another few weeks. It’s a sign that things are moving in the right direction, but not a reliable predictor of timing.
Your water breaking before contractions start is less common than most people expect. When it does happen, about 60% of women will go into spontaneous labor within 24 hours, and over 95% within 72 hours. But for many women, the amniotic sac doesn’t rupture until labor is already well underway.
Braxton Hicks contractions, the irregular tightening you may feel in the second and third trimesters, are your uterus practicing. They can become more frequent as your due date approaches, which sometimes creates confusion about whether labor has truly started.
Telling Real Contractions From Practice Ones
The key difference between Braxton Hicks and true labor contractions is pattern and persistence. False contractions are irregular, vary in intensity, and often stop when you change positions, rest, or drink water. True labor contractions keep coming regardless of what you do. They grow progressively longer, stronger, and closer together over time.
A common guideline is to call your provider or hospital when contractions come every five minutes and have been consistent for at least one hour. Sometimes, though, the only way to confirm early labor is a cervical exam to check for dilation.
What Happens to Your Cervix
Your cervix goes through two major changes before and during labor: effacement (thinning) and dilation (opening). Before labor, the cervix is typically about 4 centimeters long and firm. As labor approaches, prostaglandins and pressure from the baby soften and shorten it. Effacement is measured as a percentage: 50% effaced means your cervix has thinned to about half its original length, while 100% means it’s paper-thin.
Dilation is measured in centimeters from 0 to 10. The early (latent) phase of labor covers roughly 0 to 6 centimeters and is typically the longest and slowest part. Many people spend hours or even days in this phase with contractions that are manageable but irregular. Active labor picks up from about 6 centimeters onward, with faster dilation and more intense contractions. Full dilation at 10 centimeters means it’s time to push.
You’re generally considered to be in established labor when contractions are regular and strong, your cervix is at least 4 to 5 centimeters dilated with ongoing change, and effacement has reached around 80% or more.
When “Full Term” Actually Is
The old idea that anything between 37 and 42 weeks is simply “term” has been replaced with more precise categories, because outcomes for babies vary quite a bit across those five weeks. The current classification breaks it down like this:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post-term: 42 weeks and beyond
Babies born in the full-term window generally have the best outcomes, particularly for respiratory health. This is consistent with the fetal lung signaling mechanism: those extra weeks give the lungs time to mature and produce the surfactant that both helps the baby breathe and helps trigger labor.
Can You Encourage Labor to Start?
Membrane sweeping (or stripping) is the most evidence-backed method for encouraging labor without medication. During a cervical exam, your provider separates the amniotic membranes from the lower part of the uterus with a finger, which releases local prostaglandins. A 2020 Cochrane review of 17 studies with over 3,100 participants found that membrane sweeping increased the likelihood of going into spontaneous labor compared to simply waiting. It also cut the chance of still being pregnant past 42 weeks roughly in half. The procedure doesn’t require hospitalization and hasn’t been linked to increased risks for mother or baby in the research.
Nipple stimulation is another approach with some evidence behind it, as it triggers your body to release oxytocin. Other commonly discussed methods, like walking, spicy food, or sex, have less rigorous evidence but are generally considered safe at term. None of these techniques will force labor to start if your body and baby aren’t ready. They work best when the hormonal and physical groundwork is already in place.