ENHANCED FETAL GROWTH IN SHEEP ADMINISTERED PROGESTERONE DURING THE 1ST 3 DAYS OF PREGNANCY

Two experiments were conducted to determine whether administration of progesterone during early pregnancy affects fetal growth in sheep and if any effect is specific to the days of treatment. In the first exper iment, Merino ewes were randomly allocated to four treatment groups an d inseminated at a synchronized oestrus. Three groups received progest erone on days 1-3, 3-6 or 1-6 of pregnancy while the fourth group was untreated. Concentrations of progesterone in peripheral plasma increas ed (P < 0.05) in all treatment groups. Fetal growth (to day 74) was gr eater in all treatment groups than in the control group (P < 0.001) an d was greatest when treatments started on day 1. Pregnancy rate was no t affected by progesterone treatment on days 3-6, but was reduced (P < 0.05) when treatment began on day 1. In the second experiment, embryo s that had been exposed to either a normal (control) or a high concent ration of progesterone on days 1-3 were randomly transferred, within g roups, to recipient ewes that had or had not been treated with progest erone on days 1-3. In another group, embryos were exposed to a high co ncentration of progesterone on days 1-3 and the oviducts of the ewe we re ligated. An increase in fetal mass was observed in the recipient gr oup that had been treated with progesterone (P < 0.01) but was not obs erved in the initial group treated with progesterone. A greater fetal mass was also obtained when embryos that had been ligated in the ovidu cts of ewes treated with progesterone (P < 0.05) were transferred. Thi s effect occurred irrespective of whether the final recipients receive d progesterone. These findings indicate that progesterone supplementat ion to ewes during the first 3 days of pregnancy enhances the growth o f surviving fetuses. It is concluded that progesterone treatment or li gation of the oviducts during the first three days of pregnancy induce changes in embryo development that affect subsequent development.