Placental growth in the pig.

The growth of the porcine placenta from 38 days until term is described. Measurements were made of its area, circumference and in situ length. Placental area increased during the period of study due to increases in the uterine circumference at the sites of conceptuses. No change was found in uterine horn length or placental length with age. Fetal weight correlated well with placental area but poorly with the other parameters. Placental length was shorter in more crowded horns and showed a U shaped distribution with position within uterine horns. These results are discussed in terms of competition for space within the uterus as a cause of the within litter variation in fetal size.