Birth is a relay. The uterus, cervix, baby, and placenta each change in a way that sets up the next step, and the whole thing typically unfolds in three stages: opening the cervix, pushing the baby out, and delivering the placenta. Even with all that is known, scientists still cannot say exactly what tells a human body that today is the day.1PubMed Central. There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form Much of the timing seems to be set weeks earlier, not in the hours before the first contraction.
What Starts Labor
Throughout pregnancy, progesterone, a hormone made mostly by the placenta (the organ linking mother and baby), keeps the uterus relaxed.2PubMed. The involvement of progesterone in the onset of human labour In many animals progesterone levels fall before labor. In humans, blood levels stay high right up to birth.3PubMed. The involvement of progesterone in the onset of human labour Researchers think the uterus instead becomes deaf to progesterone.4PubMed. Mechanisms underlying “functional” progesterone withdrawal at parturition
Near term the uterus breaks progesterone down into inactive forms, so less reaches the control centers inside the muscle cells.5PubMed Central. Molecular evidence of functional progesterone withdrawal in human myometrium The cells also shift which version of the progesterone receptor they rely on, and one version switches on labor genes when progesterone is absent.6PubMed Central. Molecular evidence of functional progesterone withdrawal in human myometrium Inflammation inside the uterus can block progesterone’s calming signal as well.7PubMed. Mechanisms underlying “functional” progesterone withdrawal at parturition
Alongside this, a hormone loop between baby and placenta may act as a clock. Late in pregnancy the placenta makes rising amounts of a stress hormone called CRH.8PubMed. Cortisol blockade of progesterone: a possible molecular mechanism involved in the initiation of human labor CRH prompts the baby’s adrenal glands to make more cortisol.9PubMed. Corticotropin-releasing hormone directly stimulates cortisol and the cortisol biosynthetic pathway in human fetal adrenal cells Cortisol in turn pushes the placenta to make still more CRH, so the loop feeds itself.10PubMed. Corticotropin-releasing hormone directly stimulates cortisol and the cortisol biosynthetic pathway in human fetal adrenal cells Cortisol may also compete with progesterone in the placenta, helping silence it.11PubMed. Cortisol blockade of progesterone: a possible molecular mechanism involved in the initiation of human labor CRH levels measured weeks before birth are linked to when labor begins, which suggests a placental clock set early.12PubMed. Placental corticotropin-releasing hormone (CRH), spontaneous preterm birth, and fetal growth restriction: a prospective investigation The same loop helps mature the baby’s organs, such as the lungs, for life outside.13PubMed. Corticotropin-releasing hormone directly stimulates cortisol and the cortisol biosynthetic pathway in human fetal adrenal cells
Because the cortisol comes from the baby’s own adrenal glands, the baby partly decides when labor starts. That ties the readiness of the baby’s organs to the timing of labor, and it is one reason labor can’t simply be predicted from a due date.
How the Cervix Softens and Opens
The cervix, the narrow neck at the bottom of the uterus, is held firm by tightly organized collagen fibers.14PubMed Central. Cervical hyaluronan biology in pregnancy, parturition and preterm birth Before labor, it builds up a molecule that holds large amounts of water.15PubMed. Regulation of hyaluronan expression during cervical ripening The tissue swells with water and the collagen becomes disorganized, which makes the cervix soft and stretchy.16PubMed Central. Cervical hyaluronan biology in pregnancy, parturition and preterm birth
Immune cells also move into the cervix and release enzymes that break down collagen.17PubMed Central. Signaling Pathways Regulating Human Cervical Ripening in Preterm and Term Delivery Signaling molecules of inflammation drive those enzymes and attract more immune cells.18PubMed Central. Signaling Pathways Regulating Human Cervical Ripening in Preterm and Term Delivery Prostaglandins, hormone-like substances, boost this inflammation and help the uterus tune out progesterone.19PubMed. The role of prostaglandins E1 and E2, dinoprostone, and misoprostol in cervical ripening and the induction of labor: a mechanistic approach
The same process going off too early is one pathway to preterm birth. When it runs on schedule, the swelling and softening are what let the cervix thin and open to about 10 centimeters.
The cervix is not entirely passive. In early labor it can show bursts of muscle activity of its own.20PubMed. Active contractions of the cervix in the latent phase of labour In some women these bursts happen independently rather than in step with the main contractions.21PubMed. Active contractions of the cervix in the latent phase of labour
This is why medical induction uses prostaglandins. Gels or tablets placed near the cervix supply the same substances the body makes, kick-starting the softening described above. An unripe cervix is why some inductions take a day or more.
How Contractions Work
To push effectively, the uterus’s muscle cells must contract in unison.22PubMed Central. Alterations in gap junction connexin43/connexin45 ratio mediate a transition from quiescence to excitation in a mathematical model of the myometrium They manage this through tiny channels that pass electrical signals from cell to cell.23PubMed Central. Alterations in gap junction connexin43/connexin45 ratio mediate a transition from quiescence to excitation in a mathematical model of the myometrium These channels multiply sharply just before birth.24PubMed Central. Alterations in gap junction connexin43/connexin45 ratio mediate a transition from quiescence to excitation in a mathematical model of the myometrium A key channel protein rises toward term and peaks in normal labor.25PubMed. Expression of the gap junction protein connexin-43 is increased in the human myometrium toward term and with the onset of labor In women with prolonged labor, this protein was sparser in uterine tissue.26PubMed Central. Prolonged labour associated with lower expression of syndecan 3 and connexin 43 in human uterine tissue
Oxytocin, a hormone from the brain, then fires the contraction. It binds to receptors on uterine muscle cells.27PubMed. Oxytocin: its mechanism of action and receptor signalling in the myometrium That releases calcium stored inside the cells and draws more in from outside.28PubMed. Molecular mechanisms regulating the effects of oxytocin on myometrial intracellular calcium The calcium switches on the contraction machinery.29PubMed. Oxytocin: its mechanism of action and receptor signalling in the myometrium Oxytocin also makes that machinery more sensitive to calcium, which strengthens each squeeze.30PubMed. Multiple mechanisms involved in oxytocin-induced modulation of myometrial contractility
The hospital oxytocin drip supplies the same hormone the body makes. It only works well once the uterus has enough receptors and connections, which is why it can do little earlier in pregnancy. Doses are raised gradually because contractions need rest gaps to let the baby’s oxygen recover.
The pattern of contractions changes as the wiring comes together. Early contractions are short and irregular while coordination builds. In active labor they become longer, closer together, and stronger. Each one pulls the softened cervix up and open.
How the Baby Copes With Contractions
During a contraction, blood flow to the placenta drops by about 60%.31PubMed. The physiology of intrapartum fetal compromise at term The baby’s oxygen dips, reaching its lowest about a minute and a half after the contraction peaks.32PubMed. The effect of uterine contractions on fetal oxygen saturation It then takes about another minute and a half to recover.33PubMed. The effect of uterine contractions on fetal oxygen saturation
Healthy babies have defenses. A healthy baby with a normal placenta lowers its oxygen use during these dips and redirects oxygen-rich blood to its heart, brain, and adrenal glands.34PubMed. The physiology of intrapartum fetal compromise at term Stress hormones also surge in the baby during labor, helping it adapt to birth and life outside.35PubMed Central. Catecholamine secretion in fetal adaptation to stress
That is why the baby’s heart rate is monitored: staff watch how it responds to each contraction. Babies with weaker placentas may not recover fully between contractions, and contractions that come too close together leave less recovery time. This is why staff may slow an oxytocin drip.
The Mother’s Body During Labor
Each contraction squeezes blood from the uterus back into the mother’s circulation, so the heart pumps more blood with each beat.36PubMed. Maternal hemodynamic effects of uterine contractions by M-mode and pulsed-Doppler echocardiography This extra load grows as labor advances.37PubMed Central. Cardiac output during labour Near full dilation, the heart’s output rises from about 7.9 to 10.6 liters per minute during a contraction.38PubMed Central. Cardiac output during labour
A healthy heart handles this easily. People with known heart conditions are monitored closely in late labor, because pushing and the last centimeters are when the load peaks.
The body also makes its own pain relief. Levels of beta-endorphin, a natural painkiller, rise along with pain as labor progresses.39PubMed. Pain and plasma beta-endorphin level during labor When an epidural reduces pain, endorphin levels fall too.40PubMed. Pain and plasma beta-endorphin level during labor In one trial, acupressure reduced labor pain and raised endorphin levels in first-time mothers.41PubMed. The effects of conventional, warm and cold acupressure on the pain perceptions and beta-endorphin plasma levels of primiparous women in labor: A randomized controlled trial Endorphins take the edge off but don’t remove labor pain.
Pushing and the Pelvic Floor
Once the cervix is fully open, uterine contractions combine with the mother’s semi-voluntary pushing.42Springer Link / Annals of Biomedical Engineering. Forces Involved with Labor and Delivery-A Biomechanical Perspective Pressure of the baby’s head in the birth canal triggers a reflex that releases more oxytocin and creates the urge to push.43PubMed Central. Not Just Mechanical Birthing Bodies: Birthing Consciousness and Birth Reflexes
The pelvic floor is a group of muscles forming a sling under the pelvis, and the baby’s head must pass through it. One of these muscles can stretch to over three times its resting length.44PubMed Central. Levator Ani Muscle Stretch Induced by Simulated Vaginal Birth Others stretch to roughly two and a half times their length.45PubMed Central. Levator Ani Muscle Stretch Induced by Simulated Vaginal Birth Ordinary muscle tolerates stretch of only about one and a half times its length.46PubMed Central. On the Biomechanics of Vaginal Birth and Common Sequelae Some muscle injury during vaginal birth is therefore common, especially in the shortest, innermost muscle.
Leaking urine or a weak pelvic floor after birth often reflects it. Many people recover function over months, and pelvic floor exercises and physiotherapy help rebuild strength. Persistent symptoms are worth raising with a doctor.
The baby has its own work to do. The head turns as it moves to fit the pelvis’s changing shape, and the skull bones can overlap slightly, giving a temporary cone shape. This fit is tight in humans compared with other primates.
Delivering the Placenta and Stopping the Bleeding
After the baby is born, the uterus keeps contracting and its wall thickens.47PubMed. Dynamic ultrasonographic imaging of the third stage of labor: new perspectives into third-stage mechanisms Shearing forces tear the thin layer holding the placenta, releasing it.48PubMed. Dynamic ultrasonographic imaging of the third stage of labor: new perspectives into third-stage mechanisms Separation finishes only when the wall under the placenta reaches full thickness.49PubMed. Dynamic ultrasonographic imaging of the third stage of labor: new perspectives into third-stage mechanisms
The placenta site is fed by large blood vessels that are left open once it detaches.50PubMed Central. Pathophysiology of Postpartum Hemorrhage and Third Stage of Labor Interlacing muscle fibers around them contract and squeeze them closed.51PubMed Central. Pathophysiology of Postpartum Hemorrhage and Third Stage of Labor A uterus that doesn’t contract well is therefore the main reason for heavy bleeding after birth.
This is why staff give an oxytocin shot after birth to keep the uterus firmly contracted, and why they may rub the belly to help a soft uterus tighten. They also check that the placenta is complete, because leftover pieces stop the uterus from contracting fully.
Losing the placenta also removes the main source of progesterone. That drop is the signal for full milk production to begin, which links this stage directly to feeding.
The Baby’s First Breaths and Heartbeat Changes
Before birth the lungs are filled with fluid, not air. Shortly before birth, lung cells add channels that pump sodium out of the airspaces.52PubMed Central. Role of epithelial sodium channels in the regulation of lung fluid homeostasis Water follows the sodium, clearing most of the fluid. Squeezing during birth plays only a small part.53PubMed. Physiology of fetal lung fluid clearance and the effect of labor
The circulation reroutes too. In the womb, blood bypasses the lungs through shortcut vessels because the placenta does the breathing.54PubMed. The transition from fetal to neonatal circulation: normal responses and implications for infants with heart disease The first breaths make blood flow to the lungs rise roughly 30-fold.55PubMed Central. Transitional circulation and hemodynamic monitoring in newborn infants Clamping the cord removes the low-resistance placenta, which raises the baby’s blood pressure.56PubMed Central. Transitional circulation and hemodynamic monitoring in newborn infants The shortcut vessels are then no longer needed, and they close.57PubMed. The transition from fetal to neonatal circulation: normal responses and implications for infants with heart disease
Cord clamping timing matters. Waiting to clamp the cord lets about 30 milliliters of extra blood flow into the baby, so blood volume rises by around 12%.58PubMed. Transition from fetal to neonatal circulation: Modeling the effect of umbilical cord clamping Delayed clamping can also help keep oxygen up if breathing starts slowly.59PubMed. Transition from fetal to neonatal circulation: Modeling the effect of umbilical cord clamping
Many hospitals now wait at least a minute before clamping, and it can be discussed in a birth plan. Some situations need immediate clamping for the baby’s safety.
How Birth Starts Milk and Shapes the Baby’s Microbes
The breasts are prepared during pregnancy but held back by progesterone.60PubMed. Lactogenesis. The transition from pregnancy to lactation When the placenta comes out, progesterone drops and full milk production begins.61PubMed. Hormonal regulation of mammary gland development and lactation Prolactin and oxytocin from the brain keep milk flowing, driven by the baby suckling.62PubMed. Hormonal regulation of mammary gland development and lactation Removing milk early after birth matters for getting started.63PubMed. Lactogenesis. The transition from pregnancy to lactation
Colostrum, the first milk, is present in small amounts at first. Fuller milk usually arrives within a few days after progesterone falls, and frequent feeding helps signal the breasts. A retained placenta can delay it.
Birth route may also shape microbes. In one small study, daughters born vaginally had vaginal bacteria more like their mothers’ years later than those born by C-section.64PubMed Central. Vaginal microbiota of adolescents and their mothers: A preliminary study of vertical transmission and persistence The study was small, and C-section babies still develop healthy microbes.
Why Human Birth Is So Difficult
Human babies are born with no more than 30% of adult brain size, versus 40 to 65% in great apes.65PubMed Central. There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form Even so, human pregnancy length is about what is expected for a primate our size.66PubMed Central. There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form One idea is that pregnancy ends when the mother’s energy supply reaches its limit.67PubMed Central. There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form Still, differences between male and female pelvises point to a real tradeoff between walking upright and giving birth.68PubMed Central. There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form Nutrition, climate, and pelvic floor stability may also shape the fit.69PubMed Central. The obstetrical dilemma hypothesis: there’s life in the old dog yet
The fit with the pelvis is likely only part of the story; much of a human baby’s brain growth simply happens after birth.