Fetoplacental growth in sheep administered progesterone during the first three days of pregnancy.

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. An increase in fetal weight was observed in the final recipient group that had been treated with progesterone (P< 0.01) but not in the initial group treated with progesterone. Fetal weight was increased (P< 0.05) in the initial recipients treated with progesterone plus ligation. Placental weight did not differ between any of the treatments in either initial or final recipients, while placental volumes of chorionic membrane and maternal crypts were increased by progesterone, with and without ligation, in initial recipients (P< 0.05). The responses of fetal weight in final recipients were associated with increases in the weight and linear dimensions of specific fetal components (e.g. brain, kidney, heart, spleen, total gut, head width, thorax circumference). Proportionate increases were observed for most parameters with the exception of brain, heart and M tibialis anterior weight; adjusted least squares means indicated disproportionate increases in these of 5 per cent, 32 per cent and 26 per cent respectively. Enhanced fetal weight in the progesterone plus ligation group was associated with increased (P< 0.05) heart weight; a disproportionate increase of 39 per cent was recorded. Increased fetal weight and fetal heart, skeletal muscle and brain weight were correlated with increased volumes and surface area of the fetal trophectoderm and maternal fetomaternal syncytium in the final recipients treated with progesterone. It is concluded that alteration of the embryo's environment during the first few days of development enhances fetal growth disproportionately, in close association with increased abundance of the exchange epithelia in the placenta.