Effect of intrauterine positions on fetal brain development in the rat

The effect of the position on the horn on fetal brain development was studied in 2276 rats (248 litters), by assessing the intrauterine location of those fetuses which had the highest values ('maximal fetuses') of the following parameters: body weight, brain weight, brain DNA (cell number), brain protein and the ratios brain weight/body weight and brain protein/brain DNA. The fetuses were at term (21 days) or at 16-18 days postcoitum. The total numbers of animals in the right and in the left uterine horn were essentially the same: 4.64 and 4.54, respectively. For each parameter, the locations of maximal fetuses were, on the average, symmetrical with respect to the center of each horn. The distributions of maximal fetuses along each horn followed specific patterns, which were different for each horn but showed similar locations of peaks and depressions for various parameters. However, the differences between the peaks and depressions were not statistically significant. It is concluded that both horns are equipotent, and all the positions on the horn are also essentially equipotent for production of animals maximal with respect to parameters studied. This also suggests that occasional cases of outstanding high fetal brain development occur from causes other than a favorable position on the uterine horn.