Final answer:
During active labor, a fetus receives more oxygen during the relaxation phase between contractions. The placenta plays a critical role in fetal oxygenation, and fetal hemoglobin aids in oxygen uptake even when maternal blood oxygen partial pressures are low.
Step-by-step explanation:
A fetus receives more oxygen during the relaxation phase between contractions in labor. This is because, during contractions, the supply of oxygenated blood to the fetus is temporarily reduced. The fetus relies on the placenta for oxygen and nutrient exchange—through the umbilical cord and the chorion, the placental membrane separates maternal from fetal blood.
Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, enabling more effective oxygen uptake under these low oxygen partial pressure conditions. Labor and birth present a significant shift from the constant supply of oxygenated blood in utero to an environment where contractions intermittently reduce this supply.
The placenta is indispensable during pregnancy as it continuously supplies the fetus with nutrients and oxygen. When a contraction occurs, the blood flow to the fetus is constricted, but between contractions, there is a period of relaxation that allows blood flow to resume to normal levels, delivering oxygen to the fetus. During active labor, careful monitoring of the fetal heart rate can help understand the periods of oxygenation.