Pathophysiology of Uterine Muscle Contraction and Onset of Labor: From Myometrial Activation to Physiological Blood Loss

Labor begins when the uterus shifts from relative quiescence to coordinated contractility. Objective: This review analyzes the pathophysiology of uterine muscle contraction, biological readiness for labor and the boundary between physiological and dangerous blood loss. Methods: International guidelines and reviews on intrapartum care, postpartum hemorrhage and uterotonic use were examined. Results: Myometrial activation depends on gap-junction formation, oxytocin sensitivity, prostaglandin activity, cervical remodeling and inflammatory signaling. Discussion: Understanding contraction physiology improves interpretation of labor progress and hemorrhage prevention. Conclusion: The uterus is both the engine of birth and the main organ preventing postpartum bleeding; therefore, contraction biology is central to safe obstetrics.