Chapter 57: Fetal and Neonatal Physiology

Fetal and neonatal physiology encompasses the dramatic physiological transitions that occur before birth and during the critical early postnatal period, establishing the foundation for independent life outside the intrauterine environment. During fetal development, the placenta serves as the primary organ for gas exchange, nutrient delivery, and waste removal, allowing the fetus to remain relatively independent of its own lungs and kidneys. The fetal circulation includes unique shunts such as the foramen ovale and ductus venosus that bypass nonfunctional organs and redirect blood flow to maximize oxygen delivery to vital tissues. Upon birth, profound circulatory changes occur as the newborn transitions from placental gas exchange to pulmonary respiration, causing a rapid increase in systemic vascular resistance and closure of fetal shunts within hours to days. The chapter examines metabolic adjustments in the neonate, including the shift from maternal glucose supply to independent glucose homeostasis, activation of thermogenesis for temperature regulation, and establishment of oral feeding and nutrient absorption. Respiratory adaptations involve clearing fetal lung fluid, initiating breathing, and achieving adequate ventilation and oxygenation within minutes of birth. The chapter also addresses renal maturation, including the gradual development of glomerular filtration rate and concentrating ability, as well as hematologic changes such as the shift from fetal hemoglobin to adult hemoglobin and adjustment of blood volume. Clinical conditions reviewed include respiratory distress syndrome related to surfactant deficiency, persistent fetal circulation when normal shunt closure fails, neonatal hypoglycemia, and thermoregulatory challenges in premature infants. Understanding these transitional physiology mechanisms is essential for recognizing normal newborn adaptation and identifying pathological deviations requiring clinical intervention.