Physiology of labor; Hormonal Triggers, Stages of Labor, Cervical Ripening & Uterine Activation

The physiology of labor is a complex, beautifully coordinated process involving hormonal signals, uterine muscle activation, and cervical changes that culminate in childbirth. It’s driven by both maternal and fetal factors, and unfolds in distinct stages. 🔬 Hormonal Triggers of Labor Labor begins when a cascade of hormonal shifts prepares the uterus and cervix for delivery: Estrogen ↑ / Progesterone ↓: Estrogen rises while progesterone declines, making the uterus more sensitive to contraction signals. Oxytocin: Released by the maternal and fetal pituitary glands, it stimulates uterine contractions. It also promotes prostaglandin release, creating a positive feedback loop. Prostaglandins (PGF2α, PGE2): Produced by fetal membranes and the uterus, they soften the cervix and intensify contractions1. Fetal Cortisol: Increases late in pregnancy, enhancing estrogen production and preparing fetal organs for birth. 🧠 Stages of Labor: Physiological Breakdown 1️⃣ First Stage – Cervical Dilation Begins with regular, painful contractions. Cervix undergoes effacement (thinning) and dilation (opening from 0 to 10 cm). Uterine myometrium becomes more excitable and responsive to oxytocin. 2️⃣ Second Stage – Fetal Expulsion Strong contractions and maternal pushing help deliver the baby. Fetal head rotates and extends to navigate the pelvis. Crowning occurs as the head becomes visible at the vaginal opening. 3️⃣ Third Stage – Placental Delivery Continued uterine contractions detach and expel the placenta. Uterine involution begins to prevent hemorrhage. 4️⃣ Recovery (Fourth Stage) Uterus contracts to compress blood vessels. Cervix begins repair and lactation initiates. 🧬 Cervical Ripening & Uterine Activation Cervical tissue breaks down collagen and increases hyaluronic acid, making it soft and stretchy. The uterus forms more oxytocin receptors, increasing sensitivity to contractions. #labour