Spacing of conceptuses in the uterine horn and local effects on fetal and placental weights throughout gestation in the rat.

Longitudinal growth of the uterine horn and distances between implantation sites and the extremities of the horn were measured in 30 albino Wistar rats at Days 7, 10, 13, 16 or 22 of gestation. Growth of the uterus was most rapid over Days 13-16 but continued over Days 16-22. Distances between implantation sites and between the extremities of the uterine horn and neighbouring implantation sites were relatively even in that the coefficients of variation of these distances were 28, 32, 19, 35 and 35% at Days 7, 10, 13, 16 and 22, respectively. This indicates that an active mechanism promotes even spacing since the expected coefficients of variation given completely random spacing of conceptuses was calculated to be about 100%. Local crowding of fetuses in the uterine horn did not appear to affect fetal or placental growth except at Day 22 when there was a weak but significant correlation (r = 0.3) between fetal weight and the harmonic mean distance to neighbouring implantations.