Labor and birth as a biological and evolutionary process: from physiology to evidence-based midwifery practice

Birth is an extraordinary biological and anthropological event, where physiological, cultural, emotional, and symbolic dimensions converge. Labor and birth, in their orchestrated sequence of stages, represent the culmination of a complex evolutionary adaptation that has allowed the human species to reconcile bipedal locomotion with the development of a large brain.

It is precisely at this critical point, defined by Washburn as “the obstetric dilemma”, that the evolutionary value of the three stages of labor lies: dilation, expulsion, and afterbirth. These represent not only necessary biological steps for birth, but also key moments of interaction between the maternal body, the clinical environment, and the newborn entering the world.

The first stage: dilation

The first stage of labor, or dilating phase, marks the beginning of the active process of birth. It starts when uterine contractions reach sufficient intensity, frequency, and duration to bring about morphological changes in the cervix.

The cervix, initially firm and closed, undergoes effacement and progressive dilation until the full ten centimeters are reached. The endocrine regulation of this process involves the synergistic interaction of hypothalamic oxytocin, locally produced prostaglandins, and a system of mechanical and neurohormonal feedback known as the Ferguson reflex, through which distension of the lower uterine segment stimulates further oxytocin secretion.

The contractions themselves follow a geometric pattern determined by the organization of myometrial fibers into a “contractile trident”, the efficiency of which is now studied with uterine electromyography tools and computational models of uterine dynamics. Clinically, this stage requires careful but respectful midwifery evaluation of physiological timing, with individualized monitoring that reduces unnecessary interventionism.

The second stage: expulsion

This is followed by the second stage, called the expulsive stage, which begins with full cervical dilation and ends with the birth of the baby. In this phase, uterine contractile activity intensifies and the maternal expulsive reflex is triggered, accompanied by the spontaneous urge to push.

The fetus, guided by uterine forces and pelvic mechanics, performs a sequence of adaptive movements called cardinal movements, through which it orients and molds itself to pass through the birth canal. This sequence, precisely described as early as the 17th century by François Mauriceau, reflects the biomechanical adaptation of the fetal occiput to the obstetric curves of the maternal pelvis, evidence of the intricate balance between form and function that characterizes the evolution of human birth.

The clinical management of the expulsive stage today involves active but non-directive support of maternal effort, with an emphasis on free positioning, respected birth, and early mother–newborn contact. The contemporary approach favors non-invasive surveillance, avoiding systematic resort to episiotomy and obstetric instruments, reserving them only for selected, well-indicated situations.

The third stage: afterbirth

With the birth of the baby, the third stage of labor begins, the afterbirth, during which the uterus, thanks to tonic contractions, ensures the detachment and expulsion of the placenta and fetal membranes.

It is a crucial moment, often underestimated, but fundamental for the prevention of postpartum hemorrhage, which still represents one of the leading causes of maternal mortality worldwide. Placental separation occurs according to two main physiological modes: central (Schultze) and marginal (Duncan), depending on the initial area of separation between the basal decidua and the placenta.

Modern assistance in the afterbirth phase makes use of evidence-based protocols, such as active management of the third stage of labor, recommended by the WHO and major international bodies, which involves prophylactic administration of oxytocin, controlled cord traction, and uterine palpation to facilitate complete expulsion of the appendages.

The integration between physiology and the psychological dimension

The three stages of labor, though so distinctly described anatomically and functionally, are part of a physiological continuum in which the psychological and emotional component plays a decisive role.

Recent studies in perinatal neurobiology have shown how the affective and sensory environment in which birth occurs alters maternal and fetal production of key hormones, such as oxytocin, catecholamines, and beta-endorphins. These substances not only regulate uterine tone and pain perception, but also modulate the quality of early mother–infant interaction, influencing bonding, initiation of breastfeeding, and the newborn’s neurobehavioral development.

In conclusion, the stages of labor and birth are not only essential biological steps, but true anthropological transitions, in which the woman crosses a physiological and existential threshold.

In-depth knowledge of the mechanisms that regulate them, integrated with clinical practice grounded in evidence, human connection, and cultural awareness, represents the key to a new midwifery: scientifically sound, yet humanly oriented.