Clinical meaning
The second stage of labor begins when the cervix is fully dilated (10 cm) and ends with the delivery of the infant. During this stage, the laboring patient actively bears down with contractions to push the fetus through the birth canal. Understanding fetal oxygenation during pushing requires knowledge of the uteroplacental oxygen transfer system and the physiologic stresses placed on the fetus during each contraction and pushing effort. Fetal oxygenation depends entirely on maternal blood flow through the uterine arteries to the intervillous spaces of the placenta. Oxygen-rich maternal blood enters the intervillous space, where oxygen diffuses across the placental membrane (syncytiotrophoblast layer) into fetal capillaries within the chorionic villi. Fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin (HbA) due to its reduced binding of 2,3-diphosphoglycerate (2,3-DPG), which shifts the oxygen-hemoglobin dissociation curve to the left. This allows the fetus to extract oxygen from maternal blood even at relatively low oxygen partial pressures. During each uterine contraction, the myometrial muscle fibers compress the spiral arteries and reduce or temporarily interrupt blood flow to the intervillous space....