Effects of placental weight and maternal nutrition on the growth rates of individual fetuses in single and twin bearing ewes during late pregnancy.
Daily changes in the curved crown-rump length (CRL) of individual fetal sheep were measured using a device inserted into the fetus at about 90 days of gestation. Fetal dimensions and placental weight were measured at 142 days. Fetal growth rates were determined from changes in the observed CRL after 100 days of gestation, published data on the relationship between CRL and weight, and the observed weight at 142 days. In ewes that were adequately nourished throughout pregnancy the fetal CRL increased by about 6.5 mm per day until at least 120 days of gestation and then the rate decreased to a minimum of 3.7 mm per day by 132 days. When ewes were fed an inadequate diet between 112 and 142 days, the rate of change in fetal CRL decreased within three days from about 6.5 to 3.7 mm per day and subsequently decreased to 2.9 mm per day by 132 days of gestation. In ewes that were adequately nourished except for a period of inadequate feeding between 112 and 132 days of gestation the fetal CRL increased at similar rates to those in the second group until 132 days and then at 3.6 mm per day. Within each group placental weight was correlated positively with the total increments in CRL (observed) and weight (derived) between 112 and 142 days of gestation and with the 142 day values for CRL, girth and weight. Most differences between single and twin fetuses were attributable to differences in placental weight.