High doses of soy-derived estrogens can improve pregnancy rates in women undergoing in vitro fertilization and embryo transfer, Italian researchers have found.

Women who took 1,500 milligrams of soy isoflavones per day, along with progesterone injections, had significantly greater rates of implantation, ongoing pregnancy and delivery than women given progesterone injections with placebo, Dr. Vittorio Unfer of the Obstetrics and Gynecology Center in Rome and colleagues report.

Progesterone injections after egg retrieval are standard practice, Unfer and his team write in the medical journal Fertility and Sterility, but the role of estrogen supplementation has been controversial.

They conducted the current study to determine if high doses of plant estrogens - previously shown by the researchers to have estrogen-like effects on the uterine lining of postmenopausal women - could help support implantation of the early embryo.

The researchers randomly assigned 213 women to take soy supplementation or inactive placebo supplements after egg retrieval. The women continued to take the daily supplement until a pregnancy was confirmed or ruled out.

Among women given the supplements, 25 percent of the embryos transferred achieved implantation, compared to 20 percent among women given placebo.

Clinical pregnancy was seen in 39 percent of women on the soy estrogens and 21 percent of those on placebo.

The rate of successful delivery per pregnancy was 30 percent among women who took the active supplement compared with 16 percent for women given the placebo.

Larger studies are necessary before plant-derived estrogens can be routinely recommended for in vitro fertilization and embryo transfer, the researchers write. “Nevertheless, our findings suggest new avenues for future fertility research and treatment,” they conclude.

SOURCE: Fertility and Sterility, December 2004.