In recent years, some concern has been expressed that soy isn't healthy -- either in general or during pregnancy. The basis for this concern is that soy contains plant hormones called phytoestrogens, which are similar to the human estrogen hormone. Studies suggest that soy is quite safe during pregnancy, however, in addition to being nutritious.
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Chemicals, including hormones, have effects in the body that they exert by binding to receptors. It's possible for one chemical to mimic the activity of another, if the two molecules have similar shapes. Phytoestrogens, the plant hormones found in soy, have shapes similar to that of human estrogen. Since high levels of estrogen can increase your risk of certain kinds of cancer and other health conditions, there's been some concern that phytoestrogens might have similar effects, or might damage unborn babies.
Soy and Cancer
The many studies that have examined the relationship between soy consumption and cancer have failed to show that eating soy increases cancer risk, or any other health risk, for that matter. Further, studies actually suggest the contrary -- those who consume large quantities of soy appear to have lower rates of cancer than those who don't consume soy. This is particularly true of breast and colon cancer, notes a 1991 study by Dr. M. Messina and colleagues in the "Journal of the National Cancer Institute."
Soy and Infants
While it's difficult, from an ethical standpoint, to conduct tests on pregnant women, some studies have shown that soy consumption is perfectly safe for infants. A 2002 study in the "Journal of Nutrition" by Dr. Thomas Badger and colleagues points out that infants who consume soy formula eat the most soy per pound of body weight of any group in the United States. Because this group appears to suffer no ill effects, it's reasonable to extrapolate that soy consumption during pregnancy isn't dangerous for developing babies.
Another common concern that pregnant women sometimes have with regard to soy protein is that, since it's a common allergen, it might increase the likelihood that the baby will be born with food allergies. Drs. Michael Roizen and Mehmet Oz, in their book, "You: Having A Baby," note that there is no evidence to suggest that maternal diet influences childhood food allergies. As such, you can safely consume soy protein without worry.