We have previously shown that administration of progesterone during early pregnancy in sheep enhances fetal weight and crown–rump length. The present study examined the effect of this treatment on individual fetal organ weights and on placental growth and structure. Embryos that had been exposed to either a normal or a high concentration of progesterone on days 1–3 in initial recipient ewes were transferred at random to final recipient ewes that had or had not been treated with progesterone on days 1–3. Embryos in an additional group of ewes were exposed to progesterone on days 1–3 with oviducts of the ewes ligated.