Fetal and placental weight relationships in the rat at days 13 and 17 of gestation.

Mean fetal and placental weights were, respectively, 0.018 and 0.051 g on Day 13 and 0.376 and 0.250 g on Day 17. Fetal and placental weights within litters were weakly correlated on Day 13 (r = 0.322) but not on Day 17. Litter size was negatively correlated with placental weight on Day 17 (r = -0.485) but not with fetal weight. Male fetuses were heavier than female fetuses on Day 17 but their placentas were not significantly different. Fetuses and placentas were lighter at the ovarian end of the uterine horn on both days examined, revealing an early influence of local environmental factors on their growth.