Changes in the thickness of the uterus lining or "endometrium" during in vitro fertilization predict the likelihood of pregnancy, according to a report in the journal Fertility and Sterility.

"Good-prognosis patients will experience an endometrial response quickly," Dr. John L. Frattarelli told Reuters Health. "Patients who have a slower endometrial response have significantly worse pregnancy outcomes."

Frattarelli from Reproductive Medicine Associates of New Jersey, Somerset, and Dr. Grant D. E. McWilliams assessed how changes in endometrial thickness are associated with pregnancy in 132 women undergoing in vitro fertilization, a common treatment for infertility.

As noted, women who became pregnant experienced endometrial thickening much sooner after undergoing in vitro fertilization than did their peers who did not become pregnant. Further analysis suggested that a delay in endometrial thickening made it more difficult for the developing embryo to "implant" in the uterus wall, which is needed for normal development of the fetus.

By contrast, the thickness of the endometrium at the time of in vitro fertilization had no effect on pregnancy rates. Thus, it appears to be the change in thickness in response to the procedure that is critical.

"More research needs to be done on the endometrial response," Frattarelli said. He added that his group has done research showing that drugs which artificially increase endometrial thickness do not improve pregnancy rates.