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Childbirth, medically known as parturition, typically occurs within a week of a woman's due date, unless it is a multiple pregnancy, which tends to lead to early labor. As pregnancy approaches its final stages, several physiological changes come into play, driven by hormonal triggers that initiate labor. Progesterone, which initially inhibits uterine contractions during the early months of pregnancy, starts to taper off as pregnancy progresses into the seventh month. In contrast, estrogen levels in the mother's bloodstream continue to rise, altering the balance of these hormones. This shift, favoring estrogen over progesterone, renders the myometrium more susceptible to stimuli promoting contractions since progesterone's inhibitory effect diminishes. Furthermore, fetal cortisol levels increase by the eighth month of pregnancy, stimulating the placenta to produce more estrogen, which further counters the uterine-relaxing impact of progesterone. Some women might feel this hormonal shift as weak, irregular peristaltic contractions known as Braxton Hicks contractions or false labor. These contractions often subside with rest or adequate hydration.
An indicator of imminent labor, often associated with shorter labor, is the "bloody show." During pregnancy, a mucus plug accumulates in the cervical canal, sealing the entrance to the uterus. Approximately 1-2 days before the onset of true labor, this plug loosens and is expelled, accompanied by a small amount of blood. Simultaneously, the posterior pituitary increases its release of oxytocin, a hormone that triggers uterine contractions. As labor approaches, the myometrium becomes more responsive to oxytocin by producing more receptors for this hormone. Oxytocin then starts to induce stronger, more painful uterine contractions, which, in a positive feedback loop, stimulate the release of prostaglandins PGF2α and PGE2 from fetal membranes. Similar to oxytocin, prostaglandins enhance the strength of uterine contractions. The fetal pituitary also secretes oxytocin, further amplifying prostaglandin production. Given the crucial roles of oxytocin and prostaglandins in initiating and sustaining labor, it is common practice to use synthetic versions of these compounds through intravenous drip to induce labor when it is not progressing as needed. Lastly, the expansion of the myometrium and cervix due to the presence of a full-term fetus in the vertex (head-down) position is recognized as a stimulant for uterine contractions. The culmination of these changes triggers the regular contractions characteristic of true labor, intensifying in strength and frequency over time. The pain experienced during labor is attributed to the hypoxia (oxygen deprivation) of the myometrium during uterine contractions.
Stages of Childbirth
The journey from conception to the onset of labor encompasses distinct phases in the uterine environment. Phase 0, marked by a quiescent uterus, is associated with decreased levels of cAMP, cGMP, and MLCK activity, influenced by hormones like progesterone, relaxin, prostacyclin, and nitric oxide. As uterine activation commences in Phase 1, there is a notable upregulation of contraction-associated proteins, such as connexin-43, leading to an increase in gap junctions. The myometrial oxytocin receptor experiences a significant 50-fold increase, rendering the uterus responsive to uterotonins like oxytocin. Concurrently, there is dilation and effacement of the cervix, accompanied by cervical softening due to the rearrangement of collagen fibers and glycosaminoglycans. Hormonally, an elevated estrogen to progesterone ratio characterizes this phase. In Phase 2, spanning from the delivery of the fetus to the delivery of the placenta, the activated uterus responds to stimuli such as platelet-activating factor, endothelin, and Angiotensin II. Finally, Phase 3 involves the delivery of the placenta, with oxytocin playing a crucial role in postpartum bleeding and the involution of the uterus (Figure 16).
Figure 16 | Phases of labor and delivery | 26 The image outlines the sequential stages of childbirth. Stage 1: Dilation focuses on the crucial transformation of the cervix from undilated to over 10 cm in diameter, allowing passage of the fetal head. Stage 2: Birth details the mechanics of vaginal delivery, commencing with head presentation, followed by the rotation and delivery of the shoulders, and the subsequent emergence of the rest of the body and umbilical cord. Stage 3: Afterbirth Delivery illustrates the final expulsion of the placenta and associated membranes from the uterus following the birth of the newborn.
Cervical Dilation
Full cervical dilation to 10 cm in diameter, wide enough to allow the newborn's head to pass through, is imperative for vaginal birth. The dilation phase, which is the longest, usually spans 6–12 hours. However, this duration can significantly vary, lasting from minutes to hours, even days, depending on whether the mother has previously given birth. Typically, with each successive labor, this stage becomes shorter. The advancement of true labor follows a positive feedback loop. Uterine contractions stretch the cervix, promoting both dilation and effacement (thinning of the cervix). The cervical stretching instigates reflexive uterine contractions, further dilating and thinning the cervix. Additionally, cervical dilation stimulates the pituitary gland to release more oxytocin, intensifying uterine contractions. At the onset of labor, contractions may be sporadic, occurring every 3–30 minutes and lasting 20–40 seconds. However, as this stage nears its end, contractions can become more frequent, happening every 1.5–2 minutes and lasting a full minute.
Each contraction significantly reduces the oxygenated blood supply to the fetus. Therefore, a period of relaxation must follow each contraction. Sustained alterations in the fetal heart rate, either decreases or increases, signify fetal distress, which can result from overly forceful or lengthy contractions, hindering the restoration of oxygenated blood to the fetus. Emergency interventions like vacuum extraction, forceps delivery, or Cesarean section might be necessary in such situations. The amniotic membranes usually rupture at the end of the dilation stage due to the mounting pressure from the fetal head entering the birth canal.
Expulsion Stage
The expulsion stage commences when the fetal head enters the birth canal and concludes with the birth of the newborn. This phase typically spans up to 2 hours but can be shorter or longer, depending partly on the fetal orientation. The vertex presentation, known as the occiput anterior vertex, is the most common and facilitates vaginal birth, as the smallest part of the head, the posterior aspect or occiput, emerges first. In less than 5 percent of childbirth, the baby is in the breech presentation, with the buttocks descending first. Both legs are crossed and oriented downward for complete breech, while in frank breech presentation, the legs are oriented upward. Until the 1960s, vaginal delivery of breech presentations was common, but today, most breech births are managed by Cesarean section.
Vaginal birth significantly stretches the vaginal canal, cervix, and perineum. Traditionally, obstetricians would numb the perineum and perform an episiotomy, which involves making an incision in the posterior vaginal wall and perineum. Nowadays, perineal tears are allowed to occur naturally during birth. Episiotomies and perineal tears require suturing shortly after birth to ensure optimal healing. Although suturing a jagged perineal tear may be more challenging than suturing an episiotomy, tears generally heal more quickly, with less pain and damage to the surrounding muscles of the vagina and rectum. After the newborn's head is delivered, an obstetrician usually clears mucus from the baby's mouth and nose before their first breath. Subsequently, the rest of the baby's body follows quickly. The umbilical cord is double-clamped, and a cut is made between the clamps, concluding the second stage of childbirth.
Afterbirth
The final stage of childbirth is the delivery of the placenta and associated membranes, often referred to as the afterbirth. Once the newborn is expelled, the myometrium continues contracting, detaching the placenta from the uterine wall, which is then quickly delivered through the vagina. Subsequent uterine contractions help minimize blood loss at the placental site. The delivery of the placenta signifies the onset of the postpartum period, which spans approximately six weeks following childbirth. The mother's body gradually returns to a nonpregnant state during this time. If the placenta does not emerge spontaneously within about 30 minutes, it is considered retained, and an obstetrician may attempt manual removal. Surgical intervention may be required if this proves unsuccessful. The obstetrician must examine the placenta and fetal membranes to ensure their integrity. Fragments of the placenta left in the uterus can lead to postpartum hemorrhage. Uterine contractions persist for several hours after birth, facilitating the return of the uterus to its pre-pregnancy size in a process called involution, allowing the mother's abdominal organs to return to their pre-pregnancy positions. Breastfeeding supports this process.25
17.16: Physiology of Labor is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.
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