Millions of people all over the country are trading in their morning bagel for bacon, eggs, and cheese. Still, high-protein diets may have some negative consequences, such as increased blood pressure and a higher risk of heart disease. Now, scientists from the Colorado Center for Reproductive Medicine say they could cause trouble for women trying to get pregnant.

What they wanted to know: Could high-protein diets, such as the Atkins diet, affect women's fertility or lead to birth defects?

What they did: Researchers divided female mice into two groups. Group A was fed a diet containing 25 percent protein, considered moderately high by the American Heart Association. Group B received 14 percent protein–the average amount consumed by a person not on a high-protein diet. The mice were allowed to mate, and the resulting 174 embryos were transferred to surrogate mothers to isolate the effects of diet before conception. The researchers counted the number of embryos that developed into fetuses, the regulation of an important growth gene known as H19, and the rate of development of fetuses in both groups.

What they found: The mice that had eaten the high-protein diet had fewer pregnancies and less-healthy fetuses than those that had eaten a normal diet. Sixty-five percent of the embryos from mothers with a high-protein diet developed to the fetus stage, compared with 80 percent of those on the normal diet. Only 36 percent of the embryos from mothers on the high-protein diet had a normal regulation of the growth hormone H19, compared with nearly twice that many from mothers not on the high-protein diet. The fetuses were analyzed after 15 days, and those in the high-protein group were on average a third of a day behind the normal rate of development for that stage in the pregnancy.

What it means to you: David Gardner, the principal researcher, says that women trying to conceive should watch how much protein they are eating. Protein increases ammonium levels in the blood and possibly the reproductive tract, he says, affecting fetal implantation in the uterus and fetal development.

Caveats: Humans are not mice, of course, nor are we herbivores, as mice are. Nikos Vlahos, an assistant professor of gynecology and obstetrics at the Johns Hopkins School of Medicine says, "It is very difficult to extrapolate to humans" the results of this study... . For women trying to become pregnant, the best advice, he says, is to avoid extremes and eat a healthy balance of all types of foods.

Find out more: There haven't been many studies like this one, especially among humans. The American Society for Reproductive Medicine has some information, and amazon.com has a selection of books about diets that claim to increase fertility.

Read the article: This research was presented the annual conference of the European Society of Human Reproduction and Embryology, and there is not yet a corresponding journal article. A press release from the conference is available at: http://www.eshre.com