Fetal and Placental Circulation During Labor

Physiologic Background

A sufficient maternal blood supply to the placenta is of utmost importance to the fetus during pregnancy and labor. It has long been recognized that uterine contractions diminish uteroplacental blood flow. Contractions of myometrium compress the vessels traversing the uterine wall and increase intrauterine pressure, thus influencing the intervillous space pressure as well. The intrauterine pressure during labor usually ranges from 25 to 100 mm Hg. In contrast, the mean pressure in small arteries is 70 to 95 mm Hg and only 15 mm Hg at the venous end of the capillaries. Compression and even collapse of the myometrial vessels during labor are thus probable. Even a slight reduction in the diameter of an artery results in reduction of flow because resistance to flow is inversely proportional to the fourth power of the vessel radius, according to Poiseuille’s law. Obviously, during a contraction the uterine veins are affected first, and the restricted venous outflow results in a reduction of the pressure gradient over the placenta. With use of the radioangiographic technique in pregnant women, it has been shown that the maternal blood flow to the placenta decreases markedly during contractions. Similarly, in the rhesus monkey, diminished arteriolar jets have also been demonstrated. However, flow to the intervillous space appears to remain constant.

The intermittent decrement in the blood flow during myometrial contractions has been found to be inversely related to the increase in intrauterine pressure. During uterine relaxation after a contraction, an increase in blood flow—a reactive hyperemia—has been observed, ^, which compensates for the decreased oxygen delivery during the preceding contraction.

With the progress of labor, the peak intrauterine pressure during each contraction increases, and the time-averaged blood flow in the uterine artery diminishes ( Fig. 54.1 ). Woodbury and colleagues described a “maternal effective placental arterial pressure,” defined as the arterial pressure minus the pressure within the uterus (which opposes the inflow of maternal blood). To ensure perfusion of the placenta, the maternal central blood circulation responds to contractions by increasing both blood pressure and cardiac output. ^,

The circulation of a healthy fetus in uncomplicated labor usually remains unaffected. The umbilical circulation is relatively unreactive and does not seem to respond to the changes in intrauterine pressure or to the short-term changes in the maternal placental blood flow during contractions. During normal labor, uterine contractions are not of sufficient magnitude to negatively affect gas exchange over the placenta, and therefore they do not endanger the fetus. In labor with a pathologic course, in which uteroplacental blood flow is diminished, fetal hypoxemia can develop. In that situation, the fetus reacts with changes in heart rate, blood pressure, and blood flow. Uterine contractions can also cause a direct compression of the umbilical cord, with restriction of the umbilical blood flow, leading to changes in the fetal circulation.

Methods of Recording Human Fetal and Uteroplacental Blood Flow

Previously, most of our knowledge of uteroplacental and fetal circulation was based on animal experiments, in which invasive methods were used. Approximately 40 years ago, a noninvasive method making use of Doppler ultrasonography was introduced in the field of perinatal medicine and has made it possible to obtain data on blood flow in human pregnancies. ^,