Chronic fetal and maternal instrumentation in pregnant sheep: effect on gestation length and birthweight

The objective was to compare gestation length in chronically instrumented (laboratory) pregnant sheep (n = 131) and in the breeding flock (n = 476) that provided the experimental sheep. In the breeding flock, gestation length was normally distributed and varied between 141 and 151 days (mean = 147 ± 0.1 days). In the laboratory sheep, gestation length varied between 128 and 151 days (mean = 142 ± 1 day), and was bimodal, with 35.9% delivering preterm (