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

We have previously shown that administration of progesterone during early p regnancy in sheep enhances fetal weight and crown-rump length. The present study examined the effect of this treatment on individual fetal organ weigh ts 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 init ial 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 ad ditional group of ewes were exposed to progesterone on days 1-3 with oviduc ts 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 tr eated with progesterone. Fetal weight was increased (P<0.05) in the initial recipients treated with progesterone plus ligation. Placental weight did n ot differ between any of the treatments in either initial or final recipien ts, while placental volumes of chorionic membrane and maternal cry pts were increased by progesterone, with and without ligation, in initial recipient s (P<0.05). The responses of fetal weight in final recipients were associat ed with increases in the weight and linear dimensions of specific fetal com ponents (e.g. brain, kidney, heart, spleen, total gut, head width, thorax c ircumference). Proportionate increases were observed for most parameters wi th the exception of brain, heart and ill tibialis anterior weight; adjusted least squares means indicated disproportionate increases in these of 5 per rent, 32 per cent and 26 per cent respectively. Enhanced fetal weight in t he progesterone plus ligation group was associated with increased (P<0.05) heart weight; a disproportionate increase of 39 per cent was recorded. Incr eased fetal weight and fetal heart, skeletal muscle and brain weight were c orrelated with increased volumes and surface area of the fetal trophectoder m and maternal fetomaternal syncytium in the final recipients treated with progesterone. It is concluded that alteration of the embryo's environment d uring the first few days of development enhances fetal growth disproportion ately, in close association with increased abundance of the exchange epithe lia in the placenta. (C) 2001 Harcourt Publishers Ltd.