Fetal cardiorespiratory changes during spontaneous prelabor uterine contractions in sheep

Fetal cardiorespiratory changes during spontaneous prelabor uterine contractions (called contractures) were studied in 12 chronically catheterized fetal sheep at 120 to 143 days' gestation. During contractures the carcass blood flow increased significantly from 27 +/- 2 (SEM) to 32 +/- 3 ml/min/100 gm. There were no significant changes in combined ventricular output or in blood flow to the umbilical circulation, brain, heart, adrenal glands, gut, kidney, and lung. Fetal arterial blood pressure increased from 57 +/- 2 to 62 +/- 1 mm Hg (p less than 0.001) during contractions. There were no significant changes in fetal heart rate. In the fetal femoral artery during contractures the oxygen content decreased from 6.1 +/- 0.2 to 5.4 +/- 0.2 ml/dl of blood (p less than 0.001), and carbon dioxide tension increased significantly from 44 +/- 0.4 to 45 +/- 0.4 mm Hg (p less than 0.001). The pH did not change. The increase in carcass blood flow during contractures suggests that there was an increase in fetal skeletal muscular activity or tone. An increase in fetal skeletal muscle activity, together with a decrease in uterine blood flow could explain the small decrease in fetal oxygen content that occurs with each contracture. Fetal compression and/or changes from rapid eye movement to synchronized sleep or arousal observed during contractures are possible stimuli causing increased fetal skeletal muscle tone or activity. Since contractures periodically result in neuromuscular activity in the fetus in its protected fluid-filled environment, they may play a key role in fetal neuromuscular development by stimulating "exercising" in the fetus in utero.