12.2 Labor and Parturition
4 min read•august 1, 2024
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. and play key roles, intensifying contractions and promoting . 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 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 (labor), the (delivery of the baby), and the (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
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
Key Terms to Review (19)
Breech Presentation: Breech presentation is a term used to describe a fetal position during childbirth where the baby is positioned with its buttocks or feet facing the birth canal instead of the head. This type of presentation can complicate labor and delivery, as it poses challenges for both the mother and the baby. Understanding breech presentation is crucial because it may require special delivery methods, such as a cesarean section, to ensure the safety of both parties during the birthing process.
Cervical dilation: Cervical dilation refers to the gradual opening of the cervix during labor, allowing the baby to pass through the birth canal. This process is essential for a safe delivery and is measured in centimeters from 0 to 10, with complete dilation at 10 centimeters. Cervical dilation is a key indicator of progress in labor and can be influenced by various factors, including contractions and the baby's position.
Cesarean section: A cesarean section, often referred to as a C-section, is a surgical procedure used to deliver a baby through incisions made in the mother's abdomen and uterus. This method is employed when a vaginal delivery poses risks to either the mother or the baby, such as complications during labor, multiple pregnancies, or fetal distress. The procedure has become increasingly common and is an important aspect of modern obstetrics.
Dystocia: Dystocia refers to a difficult or abnormal labor that can impede the process of childbirth. It often results from a variety of factors, including the size or position of the baby, the shape of the mother's pelvis, or the strength of uterine contractions. Understanding dystocia is crucial, as it can lead to complications for both the mother and the infant if not properly managed during labor.
Electronic fetal monitoring: Electronic fetal monitoring (EFM) is a medical technique used to assess the heart rate and overall well-being of a fetus during labor. It involves the use of electronic devices to continuously record the fetal heart rate and uterine contractions, providing critical information to healthcare providers about how the fetus is responding to labor. This technique helps in identifying any potential distress and informs decisions regarding the management of labor and delivery.
Epidural Anesthesia: Epidural anesthesia is a regional anesthesia technique that involves the injection of anesthetic medication into the epidural space surrounding the spinal cord. This method is commonly used during labor and parturition to provide pain relief while allowing the mother to remain awake and alert during childbirth. By blocking pain signals from the lower body, epidurals enable women to experience childbirth with reduced discomfort.
Fetal heart rate: Fetal heart rate refers to the number of heartbeats per minute of a fetus during pregnancy and labor. It is a crucial indicator of fetal health, as it reflects the fetus's well-being and can signal potential complications during labor and parturition. Monitoring the fetal heart rate helps healthcare providers assess the oxygenation and overall condition of the fetus, guiding interventions if necessary.
First stage: The first stage of labor is the initial phase in the childbirth process, during which the cervix dilates and effaces to prepare for delivery. This stage is crucial as it marks the beginning of labor, characterized by regular contractions that help to thin and open the cervix, allowing the baby to move into the birth canal. Understanding this stage is essential for recognizing the progress of labor and ensuring both maternal and fetal well-being.
Oxytocin: Oxytocin is a peptide hormone produced in the hypothalamus and released by the posterior pituitary gland, playing a crucial role in social bonding, reproductive behaviors, and childbirth. This hormone is essential for initiating and regulating labor contractions during childbirth and is also important for milk ejection during breastfeeding, demonstrating its vital functions in both the birthing process and lactation.
Pelvis: The pelvis is a complex bony structure located at the lower part of the trunk, consisting of the hip bones, sacrum, and coccyx. It plays a crucial role in supporting the weight of the upper body when sitting or standing and provides attachment points for muscles and ligaments. The pelvis is also significant during labor and parturition as it forms the birth canal through which the baby passes, influencing the ease and safety of delivery.
Perineum: The perineum is a region located between the anus and the genitals in both males and females, playing a critical role in the support of pelvic organs. In the context of labor and parturition, the perineum is significant as it stretches during childbirth, facilitating the passage of the baby through the birth canal. Understanding its anatomy is essential for recognizing potential complications and managing perineal trauma during delivery.
Placenta previa: Placenta previa is a condition during pregnancy where the placenta partially or completely covers the cervix, which can lead to complications during labor. This positioning can result in bleeding and complications for both the mother and baby, especially as the pregnancy progresses and during the delivery process. Understanding placenta previa is crucial for managing labor and ensuring a safe delivery.
Postpartum hemorrhage: Postpartum hemorrhage refers to excessive bleeding that occurs after the delivery of a baby, specifically defined as losing more than 500 milliliters of blood following a vaginal birth or over 1,000 milliliters after a cesarean section. This condition can lead to serious complications for the mother, including shock and even death, and is a significant concern during the labor and delivery process.
Prostaglandins: Prostaglandins are lipid compounds derived from fatty acids that have a wide range of physiological effects in the body, particularly in the context of labor and parturition. They play a crucial role in regulating various processes such as inflammation, pain sensation, and the contraction of smooth muscle, especially in the uterus during childbirth. These compounds are produced at the site of action and influence numerous functions related to pregnancy, including cervical dilation and the initiation of labor.
Second stage: The second stage of labor is the phase in childbirth that follows the first stage and involves the active process of delivering the baby. During this stage, contractions become more intense and frequent, leading to the descent and eventual birth of the infant. This stage is crucial as it signifies that the baby is moving through the birth canal, which is facilitated by maternal pushing efforts and optimal fetal positioning.
Third stage: The third stage refers to the final phase of labor during childbirth, where the placenta is delivered after the baby is born. This stage typically occurs within 5 to 30 minutes after delivery and is crucial for the mother's recovery and overall health. Proper management of this stage is essential to prevent complications such as hemorrhage and ensure the complete expulsion of the placenta.
Uterine atony: Uterine atony is the failure of the uterus to contract effectively after childbirth, leading to inadequate muscle tone and a higher risk of postpartum hemorrhage. This condition is significant because it can cause excessive bleeding and complications for new mothers, making it a critical issue in maternal health following delivery.
Uterine contractions: Uterine contractions are rhythmic tightening and relaxing of the uterine muscles that play a crucial role in the process of labor and delivery. These contractions help to dilate the cervix and push the baby through the birth canal during childbirth. The strength, frequency, and duration of these contractions are vital indicators of labor progression and can vary significantly as labor advances.
Vertex presentation: Vertex presentation refers to the position of a fetus during labor where the top of the head is the part that is leading the way through the birth canal. This position is considered ideal for a vaginal delivery, as it aligns the fetus's head with the birth canal, facilitating an easier and safer passage during labor. Understanding vertex presentation is crucial as it impacts labor progression and delivery methods.