Developmental effects of the uterine environment: dependence on fetal sex in rats.

Body weight was recorded as a function of intrauterine position for 20-day-old male and female rat fetuses from 83 normal litters. Male fetuses located in the middle of the uterine horn were invariably heavier than males located at the extreme ovarian or cervical positions, those at intermediate positions being intermediate in size. The mean body weight of male fetuses was inversely related to the number of fetuses present in the ipsilateral uterine horn. The body weight of female fetuses appeared independent of both intrauterine position and ipsilateral uterine occupancy. For both sexes, fetal body weight was inversely related to the number of littermates in the contralateral horn. Male fetuses were significantly larger than female fetuses under all but the most adverse intrauterine conditions. Therefore, random fluctuations in sex ratio may distort the true relationshp between mean fetal weight and intrauterine position, if the two sexes are not analysed separately.