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UBC Theses and Dissertations

Circulatory and metabolic studies of normally grown and growth restricted fetal sheep before and during spontaneous labor and at delivery Tan, Weiping


To examine the fetal responses to labor, 2 groups of chronically instrumented fetal sheep were studied. The groups were separated on the basis of femoral arterial pH being greater (Group II) or less (Group I) than 7.15 at delivery. Fetal and placental weights in Group I (n = 6) were significantly lower than in Group II (n = 6), associated with a lower placental/fetal weight ratio, higher brain/liver weight ratio, higher adrenal weight and slightly shorter gestation length. In the antepartum period, Group I was hypoxemic, acidemic and hypoglycemic compared to Group U. Umbilical blood flow and fetal 0₂ delivery were also lower in Group I, but 0₂consumption was also reduced, so that there was maintenance of a balance between 0₂consumption and delivery. This was perhaps due to a programmed reduction in growth, thereby resulting in only modest hypoxemia and acidemia prior to labor onset. However, during labor Group I developed severe hypoxemia and lactic acidemia, whereas blood gas and acid-base status were maintained in Group II until delivery. In Group I, there was increased blood flow to most organs and tissues during labor; however, there was a fall in flow at delivery. In contrast, in Group II there was maintenance of perfusion to most organs which persisted through labor and delivery. In both groups, fetal 0₂delivery fell. In Group II, this was accompanied by a marked increase in 0₂extraction, so that 0₂consumption was maintained. In contrast, there was only a minimal change in 0₂extraction in Group I, with the result that 0₂consumption fell significantly at delivery. The increase in lactate concentration during labor in Group I was associated with a marked rise in fetal lactate uptake from the placenta, thus contributing to fetal lactic acidemia, perhaps as a consequence of inadequate oxygenation. The terminal fall in metabolic rate, coupled with the decreased perfusion to most fetal organs and tissues at delivery suggests that the growth restricted fetuses were severely compromised by the end of labor. They were thus less able to tolerate labor than the normally grown fetuses. The reduced placental weight may be involved in both the antepartum and intrapartum fetal compromise.

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