Fetal vascular pressure and heart rate responses to nonlabor uterine contractions.

The purpose of this study was to determine whether there are changes in fetal arterial pressure, venous pressure, or heart rate during nonlabor uterine contractions (i.e., contractures) in late gestation pregnant sheep. To do this, amniotic fluid pressure was continuously recorded using on-line computer techniques and subtracted from fetal arterial and venous pressures. With randomly selected data, there were no statistically significant changes with time in the fetal variables. However, when averaged with respect to time of the contractures, fetal arterial and venous pressures increased significantly during the contractures, and heart rate decreased. In addition, there were small but statistically significant decreases in each of the three fetal variables just before the contractures. Blocking the fetal autonomic nervous system eliminated the decreases in the variables before the contractures as well as the decrease in heart rate during the contractures, but arterial and venous pressure increased normally. Infusion of 1 liter of warmed saline into the amniotic space did not alter the fetal cardiovascular or amniotic fluid pressure changes during contractures. In summary, there are significant fetal cardiovascular changes during nonlabor uterine contractions, and these are consistent with a translocation of fetal blood from the placenta into the fetal body. Although the cause of fetal cardiovascular changes just before the contractures is unknown, they suggest but do not prove that the fetus may provide the signal for the nonlabor uterine contractions to occur.