Soy causes .
A few studies of the offspring of rodents who were fed soy isolates during pregnancy and lactation and rodents who were injected with soy isoflavones in infancy have found evidence of negative reproductive outcomes. However, there is a significant difference in the amount of estrogen to which fetal rats and fetal humans are normally exposed. Fetal rats are normally exposed to very small quantities of estrogen in utero whereas human fetuses are normally exposed to large quantities of estrogen in utero - this is without any soy consumption on the mother's part. It has been suggested that rats, unlike humans, are not meant to be exposed to significant quantities of estrogen in utero and their reproductive organs are, therefore, more biologically sensitive to the effects of isoflavones. Whatever the mechanism for the poor outcomes for the rodents, these results have not been replicated in humans. In fact, research on human infants exposed to soy in utero and during infancy has found no statistically significant differences in sexual development or reproductive health other than one study that found slightly longer menstrual periods in women who were fed soy formula as infants. Clinical studies of infants fed soy formula, arguably the infants with the highest exposure to soy, have found no hormonal defects, no increase in infertility and normal sexual maturation.