Childbirth is a coordinated cascade of hormonal, muscular, and mechanical events that moves a baby from the uterus through the birth canal and into the outside world. The process typically begins when a shift in the balance between progesterone and estrogen lifts the chemical brake that has kept the uterus quiet for months, allowing contractions to begin. From that point, labor unfolds in three recognized stages, each with its own biology, and the baby undergoes a remarkable physical and physiological transformation simply by being born.
What Actually Triggers Labor
For most of pregnancy, progesterone acts as a powerful calming signal to the uterine muscle, suppressing contractions and maintaining what researchers call “uterine quiescence.” Near the end of pregnancy, this progesterone block weakens while estrogen’s influence grows. The shift amplifies the uterus’s sensitivity to two key contraction-promoting signals: oxytocin (sometimes called the “labor hormone”) and prostaglandins, which soften the cervix and stimulate the muscle to contract rhythmically.1PubMed Central. The hormonal control of parturition
The baby plays an active role in starting its own birth. The fetal adrenal glands ramp up cortisol production in the final weeks, and this cortisol boosts prostaglandin output in the membranes surrounding the baby. Mechanical stretch of the uterus as the baby grows also contributes, essentially helping to “activate” the uterine muscle for labor.2PubMed. Fetal endocrine signals and preterm labor No single hormone flips a switch. Instead, changes in estrogen, progesterone, prostaglandins, oxytocin, cortisol, and corticotrophin-releasing hormone all interact in what one research group described as factors that “play in harmony” to initiate labor.3PubMed. The onset of human parturition
The fetal membranes, the “bag of waters,” also prepare for labor well before contractions start. Researchers have found a zone of altered tissue directly over the cervix where the membranes weaken through programmed changes in enzyme activity and cell death. This zone is essentially pre-scored to rupture, whether that happens before or during active labor.4PubMed. Biochemistry of fetal membranes rupture
The First Stage of Labor
Once contractions establish a regular pattern, the cervix begins to dilate. The cervix has spent the pregnancy as a firm, closed barrier at the base of the uterus, and now it needs to soften, thin out (a process called effacement), and open to roughly ten centimeters to let the baby’s head pass through. This first stage is by far the longest. For a first-time birth it can last twelve hours or more, and it is often divided into an early phase of gradual dilation and an active phase when dilation speeds up.
Each contraction works in two ways: the upper portion of the uterus tightens and pulls upward, while simultaneously pushing the baby downward against the cervix. The baby’s head itself acts as a mechanical wedge helping the cervix open. Between contractions, the muscle relaxes but does not fully return to its previous length, so progress accumulates over time. Prostaglandins continue to soften cervical tissue throughout this stage, and oxytocin levels climb, making contractions progressively stronger and closer together.
How the Baby Navigates the Birth Canal
The second stage of labor begins once the cervix is fully dilated and ends when the baby is delivered. This is the stage people picture when they think of birth: the mother pushes, and the baby emerges. But what is actually happening inside the pelvis is a surprisingly intricate sequence of rotations.
The traditional teaching in English-language obstetrics lists seven “cardinal movements” the baby makes during delivery: engagement, descent, flexion, internal rotation, extension, external rotation, and expulsion. An alternative framework used in German and older English literature condenses these into four rotational movements, setting aside engagement, descent, and expulsion as separate from the rotations themselves.5AJOG MFM. There are 4 cardinal movements in labor Regardless of how you count them, the key idea is the same: the baby’s head enters the pelvis typically facing sideways, then tucks its chin (flexion), rotates to face the mother’s back (internal rotation), tips backward as it clears the pubic bone (extension), and finally rotates outward again after the head is free so the shoulders can follow.
These rotations are not conscious. They are driven by the shape of the pelvis and the forces of contraction and maternal pushing, with the baby’s head essentially following the path of least resistance through a bony canal that changes shape at different levels.
Head Molding
A newborn’s skull is not a single rigid bone. It consists of several plates connected by flexible seams called sutures and soft spots called fontanelles. During the second stage of labor, the pressure of the birth canal causes these plates to overlap slightly, temporarily reshaping the head into a more elongated form. MRI studies have confirmed that all babies studied showed overlapping sutures during labor, even when physical examination after birth revealed a deformed head shape in only some of them, demonstrating how effectively the skull springs back.6PLOS ONE. Three-dimensional magnetic resonance imaging of fetal head molding and brain shape changes during the second stage of labor
This molding is functionally important. Biomechanical modeling has shown that a deformable fetal head reduces the forces on the pelvic floor muscles by about 17% compared with what a rigid head of the same size would generate.7PubMed. Study on the influence of the fetus head molding on the biomechanical behavior of the pelvic floor muscles, during vaginal delivery In other words, the baby’s flexible skull protects both the baby and the mother. The overlapping tends to be greatest in the front-to-back direction, producing the classic elongated “sugarloaf” head shape some parents notice in the hours after birth. It resolves on its own as the sutures settle back into position.
The Body’s Built-In Pain Response
Labor is painful, but the body has its own partial countermeasure. Pregnant women carry higher circulating levels of beta-endorphin, the body’s natural opioid-like chemical, compared with non-pregnant women. As labor progresses, beta-endorphin levels continue to rise.8PubMed. Beta-endorphin levels during pregnancy and labor: a role in pain modulation? This does not eliminate pain, but the pattern researchers have observed is telling: women report that the pain between contractions increases faster than the pain during contractions, suggesting the endorphin surge may specifically help with tolerating the acute peaks of each contraction.
The stress hormones cortisol and beta-endorphin also surge in the baby during a vaginal delivery, with umbilical cord cortisol levels measured after vaginal birth significantly higher than after cesarean delivery.9Gynecol Obstet Reprod Med. Evaluation of Maternal and Fetal Stress Hormones During the Process of Birth Far from being harmful, this hormonal surge is thought to prime the newborn for the transition to life outside the womb, helping with lung fluid clearance and metabolic adaptation in the first minutes after birth.
Delivering the Placenta
Most people think of birth as over when the baby arrives, but the third stage of labor, delivery of the placenta, still has to happen. After the baby is born, the uterus continues to contract. These contractions shear the placenta away from the uterine wall, and the placenta is typically delivered within five to thirty minutes. The same contractions also compress the blood vessels at the former attachment site, which is critical for preventing excessive bleeding.
Occasionally the placenta does not detach or deliver normally. The three main reasons are failed contraction of the muscle behind the placenta (the placenta stays stuck because the uterus is not squeezing it off), a detached placenta that gets trapped behind a closing cervix, and a small area where placental tissue has grown abnormally deep into the uterine wall.10Elsevier / PubMed Central. The retained placenta A retained placenta requires medical intervention and is one reason delivery teams monitor the third stage closely.
The Baby’s First Breath and Circulatory Overhaul
The single most dramatic event in a newborn’s life happens in the first seconds after birth: the transition from getting oxygen through the placenta to breathing air. Inside the womb, the baby’s lungs are filled with fluid and receive only a small fraction of the heart’s blood output. The moment the baby begins to breathe, that fluid is cleared and the lungs fill with air, establishing what is needed for gas exchange to start.11PubMed Central. Cardiopulmonary changes with aeration of the newborn lung
Lung aeration sets off a chain of cardiovascular changes. As the lungs inflate, resistance in the pulmonary blood vessels drops sharply, and blood suddenly floods into the lungs. This increased blood flow to the lungs raises the volume of blood returning to the left side of the heart, which in turn raises blood pressure and changes the flow dynamics through two fetal bypass routes: the oval foramen (a hole between the upper heart chambers) and the ductus arteriosus (a vessel connecting the pulmonary artery to the aorta). Over the following hours and days, both close, converting the fetal circulatory pattern into the separate pulmonary and systemic loops that will serve the person for life.12Neonatology. Measuring Physiological Changes during the Transition to Life after Birth
Why Cord Clamping Timing Matters
The timing of umbilical cord clamping interacts directly with this transition. If the cord is clamped before the baby has started breathing, the baby suddenly loses the blood returning from the placenta without yet having established the lungs as a replacement source of blood flow. This causes a drop in heart output and a spike in blood pressure as the body scrambles to compensate.13Neonatology. Physiological-Based Cord Clamping: When the Baby Is Ready for Clamping If the cord stays intact until the baby is breathing and the lungs are aerated, pulmonary blood flow smoothly takes over as the new source of return to the left heart. This is the physiological basis behind the growing clinical practice of delayed cord clamping.
Microbiome Seeding During Vaginal Birth
A less visible but increasingly studied aspect of vaginal birth is what the baby picks up on the way out. As the baby passes through the birth canal, it is coated with the mother’s vaginal and intestinal bacteria. This microbial exposure is now understood as an early inoculation that shapes the baby’s developing gut microbiome, with measurable differences between babies born vaginally and those born by cesarean section.14PubMed Central. The Maternal Infant Microbiome: Considerations for Labor and Birth
Animal research has shown that the specific composition of the vaginal microbial community transferred during birth has lasting effects on the offspring’s metabolism, immune function, and brain development.15Nature Communications. The composition of human vaginal microbiota transferred at birth affects offspring health in a mouse model This line of research is still young, and whether practices like swabbing cesarean-born babies with vaginal bacteria produce meaningful long-term benefits in humans remains an open question. But the finding that mode of delivery influences the microbiome from the very first moments of life has added a new dimension to understanding what happens during birth.
How Cesarean Delivery Changes Things for the Baby
When a baby is delivered by cesarean section, it bypasses the birth canal entirely, and that changes several aspects of the neonatal transition. One of the most studied differences involves the lungs. During vaginal delivery, the compression of the baby’s chest in the birth canal and the hormonal surge from labor both help clear fluid from the lungs. A cesarean-born baby, especially one delivered before labor begins, misses some or all of these signals.
Research has shown that babies born by cesarean section had a mean lung volume of only about 20 mL per kilogram of body weight compared with roughly 33 mL per kilogram in vaginally born babies, a difference attributed to excess retained lung fluid.16PubMed Central. Effects of delivery by caesarean section on lung mechanics and lung volume in the human neonate While elective cesarean delivery reduces the risk of birth-related trauma and meconium aspiration, it increases the risk of respiratory difficulties such as transient rapid breathing and, in rarer cases, surfactant deficiency. The last few weeks of pregnancy and the onset of spontaneous labor appear to trigger hormonal changes that prepare the fetal lungs to clear fluid rapidly through sodium-absorbing channels in the lung tissue, and skipping that preparation has measurable consequences.17PubMed. Respiratory transition in infants delivered by cesarean section
For the vast majority of cesarean-born babies, these respiratory issues are transient and resolve within hours or days. But the biology underscores how finely the process of vaginal labor and delivery is tuned to prepare the newborn for independent life.
Why Human Birth Is Unusually Difficult
Compared with most mammals, human birth is remarkably tight and time-consuming. The classic explanation, proposed in 1960 by anthropologist Sherwood Washburn, is that the human pelvis represents a compromise between two competing demands: a wide pelvis makes birth easier, but a narrower pelvis makes upright walking more efficient. This idea, known as the “obstetrical dilemma,” has been debated extensively. Recent biomechanical studies suggest that pelvic width actually has little effect on walking efficiency, so the constraint on the birth canal may not come from locomotion in the way Washburn imagined.18PubMed Central. Evolution of the human pelvis and obstructed labor: new explanations of an old obstetrical dilemma
An alternative explanation focuses on the pelvic floor’s role as structural support. A wider pelvis would make birth easier, but it would also mean weaker support for the weight of the internal organs and the large human fetus during the unusually long human gestation. In this framing, the constraint is not walking versus birth but rather pregnancy support versus birth. Additionally, bipedalism appears to have limited how much the pubic symphysis, the joint at the front of the pelvis, can loosen during pregnancy. In many other mammals, this joint opens dramatically to widen the birth canal, but in humans it remains relatively rigid.
The tight fit has another consequence: the baby must rotate as it passes through, emerging facing away from the mother. This is strikingly different from most primates, where the baby typically comes out facing the mother and can be easily caught and guided. The human pattern places a premium on having assistance at delivery, which may be why attended birth appears to be a deeply rooted human behavior.19PubMed Central. Primate pelvic anatomy and implications for birth That said, the picture is not as uniquely human as once thought. Observations of chimpanzee births have documented the same head-first, face-down emergence with post-delivery rotation, suggesting some elements of the “rotational” birth pattern predate bipedalism.20PubMed Central. Mechanism of birth in chimpanzees: humans are not unique among primates
Pelvic Loosening and the Role of Relaxin
During pregnancy the body does make some accommodations to ease delivery. The hormone relaxin, which rises during pregnancy, induces loosening of the pelvic ligaments and joints.21PubMed. Circulating levels of relaxin are normal in pregnant women with pelvic pain This increased flexibility, particularly at the sacroiliac joints and pubic symphysis, allows the pelvis to expand somewhat during delivery. It is also the reason many pregnant people experience pelvic or lower-back pain in the later months: the same loosening that helps with birth also makes the pelvis less stable for everyday movement.
The degree of pelvic flexibility varies between individuals, and research has not shown a straightforward relationship between relaxin levels and pelvic pain, meaning some women with high relaxin have no discomfort while others with normal levels experience significant symptoms. The ligaments typically firm back up in the weeks and months after delivery, though for some women the process takes longer.
What Happens to Milk Production After Delivery
The same hormonal shift that triggers labor also sets up breastfeeding. Throughout pregnancy, high levels of progesterone and estrogen promote breast tissue development but simultaneously suppress milk production. Once the placenta is delivered and those hormone levels plummet, the brake on milk production is lifted, allowing prolactin to stimulate the breasts to begin producing milk in volume. This is why colostrum, the thick early milk, is available in small amounts before birth, but the larger volume of mature milk typically “comes in” two to four days after delivery.
Roughly 10 to 15% of mothers experience low milk supply, a problem that can stem from retained placental fragments keeping progesterone elevated, hormonal disorders, breast tissue insufficiency, or other factors.22Karger. Causes of Low Milk Supply: The Roles of Estrogens, Progesterone, and Related External Factors Understanding that milk production is hormonally gated by the same events as labor and placental delivery helps explain why cesarean delivery, retained placenta, or certain endocrine conditions can affect breastfeeding and why the timing of the first feeding matters.