Influences on fetal and placental weights during mid to late gestation in prolific ewes well nourished throughout pregnancy.

This study investigated associations between fetal and placental weights from 85 to 130 days gestation in 49 fetuses from 21 ewes of a prolific genotype used as an experimental model of intrauterine growth retardation. The proportion of variation in fetal weight explained by placental weight increased from zero at 85 days to 91% (residual standard deviation (RSD) = 260 g) at 130 days. Overall, stage of pregnancy plus placental weight accounted for 96% of fetal weight variation (RSD = 212 g). Litter size and number of fetuses per uterine horn also influenced individual fetal weights. Gestational age, litter size, placental weight per ewe, and liveweight and condition score of ewes during early to mid gestation (initial LW and CS) explained 99.5% of the variation in fetal weight per ewe (RSD = 236 g). Most variation (86%) in placental weight was explained by stage of pregnancy, litter size, number of placentomes, and initial LW and CS (RSD = 53 g). Placental weight per ewe was influenced by stage of pregnancy, litter size and initial ewe LW and CS (R2 = 0.97; RSD = 89 g). The association of fetal and placental weights with initial ewe LW was positive, and with initial CS was negative. The results show that in the absence of overt nutritional restriction of pregnant ewes, fetal and placental weights are tightly coupled during late gestation and ewe fatness during early pregnancy is inversely related to placental and fetal weights. They demonstrate that placental weight explains most of the variation in fetal weight in the present intrauterine growth retardation model.