Estimation of the developmental age of the bovine fetus and newborn calf

Jersey cows from several herds provided 97 fetuses and 24 calves at fixed gestational intervals between 80 and 290 days after conception. The fetuses and calves were killed, weighed and measured and, after dissection, the sizes and weights of a range of skeletal and soft tissues were recorded. Six morphological measurements emerged as most suitable for the determination of developmental age in the normal fetus. By plotting their mean values and 95 per cent tolerance limits, the rates of growth and the variability of each measurement were defined. Long bone length was the most useful single measurement for predicting the developmental age of the fetus. Brain weight, bodyweight, crown-anus length and long bone length showed curvilinear growth patterns; age prediction equations derived from these measurements are complicated to use and additional simplified formulae have been derived. The number of appendicular ossification centres also had predictive value, but it could not be used to determine fetal developmental age between 100 and 160 days gestation.