Feb. 10, 2003 -- The difference between a healthy pregnancy and an unsuccessful in vitro fertilization (IVF) attempt may be less than few nanograms of estrogen. New research shows even the tiniest variations in a woman's estrogen levels may help explain why up to 70% of healthy embryos fail to implant themselves in the uterus.
A study in today's online edition of the Proceedings of the National Academy of Sciences suggests that estrogen plays an important role in determining the window of opportunity for an embryo to attach itself to the uterine wall and begin development.
Although a small amount of estrogen is needed to make the uterus receptive to the embryo, researchers found slightly too much of the hormone can alter the necessary genetic process at the implantation site in the uterus and can drastically shorten the fertility window.
Women undergoing IVF procedures tend to have high estrogen levels because of the drugs used to stimulate egg production. But a woman's estrogen levels may also be affected in other ways, such as the foods she eats, environmental toxins, or ethnic differences.
In the study, researchers removed the ovaries of female mice to stop the normal production of estrogen and progesterone, which are needed for successful embryo implantation. They later gave the mice daily injections of progesterone and one low dose of estrogen followed by a second high or low dose after embryo transfer. They found that a single second dose of either 3, 10, or 25 nanograms of estrogen was capable of inducing implantation.
But while the lowest dose extended the window of opportunity in mice to up to four days, increasing the dose to 10 nanograms shortened it to less than 24 hours.
"The most exciting aspect of this investigation is that a very narrow range of estrogen levels can alter embryo implantation and gene expression," says researcher Sudhasu K. Dey, PhD, of the Division of Reproductive and Developmental Biology at Vanderbilt University in Nashville, Tenn., in a news release.
Dey says altering the dose of progesterone didn't seem to have any effect on uterine receptivity.
Although replicating the results of this study in humans may be difficult, researchers say that the findings suggest that careful regulation of estrogen levels may offer a new way to improve the success of IVF procedures.