The effects of gestational age and chronic fetal decapitation on arterial blood pressure in the pig fetus.

Blood pressure was measured in anaesthetized pig fetuses decapitated at 40-43 days of gestation and in intact fetuses between 35 and 112 days of gestation (term is 114 days). In the intact fetuses arterial blood pressure increased significantly from 0.8 +/- 0.1 kPa (mean +/- SEM) at 35 days to 5.8 +/- 0.2 kPa at 112 days (P less than 0.05). The arterial blood pressure of decapitated fetuses was similar to that of intact fetuses at 70 days of gestation (2.7 +/- 0.4 kPa vs. 2.5 +/- 0.1 kPa, respectively) but did not change with increasing gestational age thereafter. Hence in late gestation (greater than 90-100 days) the arterial blood pressure of the decapitated fetuses was significantly less than that of intact fetuses (P less than 0.05). These observations demonstrate that the control of blood pressure in the pig varies with gestational age and suggest that the developmental changes occurring after about 100 days gestation require tissues within the head.