12.2 Labor and Parturition

Labor and parturition mark the culmination of pregnancy, involving complex hormonal and physiological changes. The process begins with the maturation of the fetal HPA axis, triggering a cascade of events that prepare the mother's body for childbirth.

The three stages of labor - dilation, delivery, and placental expulsion - are driven by positive feedback loops. Oxytocin and prostaglandins play key roles, intensifying contractions and promoting cervical dilation. After birth, the newborn undergoes rapid adaptations to extrauterine life.

Labor Initiation and Regulation

Hormonal Changes

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• Labor initiates around 40 weeks of gestation due to a complex interplay of hormonal changes in the mother and fetus
• The fetal hypothalamic-pituitary-adrenal (HPA) axis matures leading to increased cortisol production, which stimulates the placenta to produce more estrogen and less progesterone
• The shift in the estrogen-to-progesterone ratio promotes the production of prostaglandins, which soften the cervix and increase its sensitivity to oxytocin

Physiological Changes

• The uterine muscle becomes more excitable due to the increased estrogen levels and the formation of gap junctions between myometrial cells, allowing for coordinated contractions
• Oxytocin, released from the maternal posterior pituitary gland, stimulates uterine contractions and is involved in the positive feedback loop that maintains labor
• Mechanical factors, such as the pressure of the fetal head on the cervix, also contribute to the release of oxytocin and prostaglandins further promoting labor progression (Ferguson reflex)

Stages of Labor

First Stage

• Labor is divided into three stages: the first stage (labor), the second stage (delivery of the baby), and the third stage (delivery of the placenta)
• The first stage of labor is further divided into three phases:
  • Latent phase: characterized by mild, irregular contractions and gradual cervical effacement and dilation up to 3-4 cm
  • Active phase: contractions become more regular, intense, and frequent, leading to rapid cervical dilation from 4-7 cm
  • Transition phase: the most intense phase, with strong, frequent contractions and complete cervical dilation to 10 cm

Second Stage

• The second stage of labor begins with complete cervical dilation and ends with the delivery of the baby
• Involves the descent of the fetus through the birth canal, aided by maternal pushing efforts
• The fetus undergoes cardinal movements (engagement, flexion, internal rotation, extension, external rotation, expulsion) to navigate the pelvis

Third Stage

• The third stage of labor involves the separation and delivery of the placenta (afterbirth)
• Typically lasts 5-30 minutes after the baby's birth
• Uterine contractions continue to help separate the placenta from the uterine wall and expel it through the vagina

Positive Feedback in Labor

Oxytocin Loop

• The primary positive feedback loop involves the release of oxytocin from the maternal posterior pituitary gland, which stimulates uterine contractions
• As the uterine contractions increase in intensity and frequency, they cause the cervix to dilate and the fetal head to descend, putting pressure on the cervix and vaginal wall
• This pressure stimulates stretch receptors in the cervix and vagina, sending signals to the hypothalamus to release more oxytocin, further intensifying the contractions
• The increased contractions lead to greater cervical dilation and fetal descent, creating a self-amplifying loop that maintains and progresses labor

Prostaglandin Loop

• Another positive feedback loop involves the production of prostaglandins, which soften the cervix and increase its sensitivity to oxytocin, leading to more effective contractions
• The increased contractions stimulate further prostaglandin production, creating a self-sustaining cycle that promotes cervical ripening and labor progression

Newborn Adaptation to Extrauterine Life

Respiratory Adaptations

• The first breath is triggered by a combination of factors, including sudden exposure to cold air, increased carbon dioxide levels, and decreased pH in the newborn's blood
• The lungs expand, and the alveoli fill with air, establishing functional residual capacity
• Pulmonary vascular resistance decreases, allowing for increased blood flow to the lungs and efficient gas exchange

Cardiovascular Adaptations

• The foramen ovale, a shunt between the right and left atria, closes functionally due to increased left atrial pressure
• The ductus arteriosus constricts in response to increased oxygen tension, directing blood flow to the lungs
• These changes establish the adult pattern of circulation, with the lungs receiving the entire cardiac output

Thermoregulation and Metabolic Adaptations

• Brown adipose tissue metabolism is activated to generate heat and maintain body temperature (non-shivering thermogenesis)
• Shivering thermogenesis and vasoconstriction of cutaneous blood vessels help prevent heat loss
• The newborn transitions from continuous glucose supply via the placenta to intermittent feeding, requiring the mobilization of glycogen stores and the initiation of gluconeogenesis
• The newborn's ability to regulate blood glucose levels matures over the first few days of life

Gastrointestinal Adaptations

• The newborn's gastrointestinal tract becomes colonized with beneficial bacteria (gut microbiome), aiding in digestion and immune system development
• Digestive enzymes (lactase, lipase) and hormones (cholecystokinin, motilin) are secreted to facilitate the digestion and absorption of nutrients from breast milk or formula
• The newborn's ability to coordinate sucking, swallowing, and breathing enables effective feeding