Gestational changes in pig placental weight, area and DNA content in the second half of pregnancy

Weight and macroscopic surface area of the pig placenta reached a maximum at day 101 and decreased thereafter. However, between day 101 and day 111 (full term 113/115) the epitheliochorial tissue layer increased in area due to the continuing formation of folds. Although lightweight placentae had small macroscopic surface areas, they possessed larger areas per unit weight throughout the second half of gestation. From day 70 to day 92 the lightest placentae, when compared with their heaviest litter mates, had from 15 to 150 per cent more macroscopic surface area per unit weight. In the middle third of gestation there was a phase of rapid cell division, in consequence of which mean cell size was reduced. By the beginning of the last third of gestation mean cell size had reached a steady value, with no demonstrable change thereafter. Cell multiplication continued until day 101 when growth in weight had ceased.