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Objective: Intrauterine growth restriction in an ovine model occurs after exposure to environmental heat stress for 80 days beginning at 35 days' gestation. Our objective was to determine whether intrauterine growth restriction is reversible on removal of the heat stress after only 55 days of exposure; that is, does a brief exposure at a critical point of development suffice?, Study Design: Five pregnant ewes were exposed to heat stress beginning at 35 days' gestation and were removed after 55 days of exposure. Five ewes in a control group were studied as contemporaneous controls and added to data from 37 ewes in a control group previously studied. Serial fetal biometric ultrasonographic measurements (biparietal diameter, abdominal circumference, femur length, and tibia length) were obtained beginning at 50 days' gestation. Growth curves were calculated for each parameter, and comparisons were made between fetuses in the group exposed to heat stress for 55 days and 42 fetuses in the control group and 4 fetuses from a previous study that were exposed to heat for 80 days. Regression lines, 95% confidence intervals, and slopes were determined for each study group., Results: Both the 55-day and the 80-day heat exposure groups showed a significant reduction in fetal and placental weights compared with the control group. Animals in the 80-day group had significantly lower fetal and placental weights than the animals in the 55-day group (P <.05). Indexes of somatic growth (abdominal circumference, femur length, and tibia length) for the control group were significantly greater than those of either the 55-day group or the 80-day group (P <.001). Asymmetric growth restriction was evident in both heat groups by a biparietal diameter/abdominal circumference ratio that was significantly higher than in the control group (P <.004 for the 55-day group and P <.001 for the 80-day group). The slopes for somatic parameters (abdominal circumference, femur length, and tibia length) versus time became significantly different between the control and 55-day groups at 77, 101, and 80 days' gestation, respectively. The 55-day group had abdominal circumference and femur length measurements that were significantly greater than those in the 80-day group., Conclusion: The fetuses in the 55-day and 80-day groups reflect a pattern of asymmetric intrauterine growth restriction. Our findings suggest that the initial insult affecting fetal and placental growth occurs early in gestation, but removal of fetuses after only 55 days of exposure significantly reduces the degree of fetal growth restriction compared with that found in those fetuses exposed for 80 days.

Objective: With an ovine model in which growth restriction was induced by exposure to heat stress, our aims were as follows: (1) to describe the normal gestational age-related changes in Doppler velocimetry in the ovine fetal aorta and umbilical artery and (2) to compare Doppler velocimetry between heat-stressed and non-heat-stressed fetuses., Study Design: Five ewes were exposed to heat stress for 55 days beginning at 35 days' gestation. Five ewes were not exposed and served as controls. Aortic and umbilical artery pulsed Doppler velocimetry was obtained, including the systolic/diastolic ratio, pulsatility index, and resistance index. Data were obtained between 50 and 120 days' gestation. Linear regression analysis was used to analyze gestational age-related changes in velocimetry. Comparison of mean index values between heat-stressed and non-heat-stressed fetuses were made with analysis of variance., Results: Heat-stressed fetuses demonstrated significantly higher systolic/diastolic ratios and pulsatility index values for both umbilical artery (p < 0.025; p < 0.033) and aorta (p < 0.017; p < 0.022) compared with controls. The umbilical artery and aortic resistance index values were not different between groups (p < 0.079; p < 0.28). The slopes for each of the Doppler index values were negative in both normal and heat-stressed fetuses. Umbilical artery and aortic end-diastolic flows remained absent through the first 70 days of gestation in both groups., Conclusion: Doppler velocimetry index values decrease with increasing gestational age, reflecting decreased placental bed vascular resistance. The higher Doppler index values seen in the heat-stressed group are consistent with increased placental vascular resistance. The normal absence of diastolic flow until 70 days' gestation is similar to the pattern described in humans.