Depression of uterine blood flow in response to cord compression in sheep

Seven conscious chronically cannulated ewes and their fetuses were studied in the third trimester of pregnancy. Compression of the umbilical cord for 4 min, to a degree simulating the occurrence in clinical obstetrics, not only produced the expected fetal hypoxia, acidemia, and cardiovascular effects but also significantly reduced uterine blood flow. In a ewe with twin fetuses, compression of the cord to only one fetus decreased the flow to that horn and had no effect on flow to the other horn. It is postulated that the reduction in uteroplacental flow was due to stimulation of α-adrenoceptors in the maternal placental vasculature by catecholamines of fetal origin, or the effects of increased tissue-fluid pressure in the fetal placenta, or a combination of these mechanisms.