Fetal and placental growth in young, primiparous and old, multiparous rats.

Fetal and placental weights and peripheral blood progestagen concentrations were examined on days 13, 16 and 22 of gestation in young (3-5 month) nulliparous and old (9-12 month) multiparous, albino Wistar rats. There was no difference in fetal weights at any stage examined whereas placental weights were significantly heavier in the old rats at days 16 (71%) and 22 (38%). Blood progesterone concentrations were similar in the two groups at days 16 and 22 although concentrations of the less biologically active progestagen, 20 alpha-dihydroprogesterone, were higher (61%) at day 16 in the old rats. The relative hypertrophy of the placentas in the old rats could not be explained by reduced litter size or progesterone concentrations: it may represent a compensatory effect in response to a less favorable uterine environment.