The condition of the lining in the uterus may be the difference between pregnancy success or failure in patients undergoing in vitro fertilization (IVF), specifically, its thickness according to a new study from Northwestern University.¹

Measuring the thickness of the uterine lining, known medically as the endometrium (end-oh-MEE-tree-um), relates to determining how receptive it might be to the pending implantation of an embryo in IVF procedures. "Endometrial receptivity refers to a physiological state of the endometrium that is optimal for embryo implantation," wrote Xingqi Zhang, PhD, in the section of Reproductive Endocrinology and Infertility at Northwestern, and colleagues.

Obviously, a successful implantation is a prerequisite to a successful pregnancy. But so far, finding a biological indicator in the body that can tell doctor's that a woman's uterus is ready or not ready to receive an embryo has been "elusive", Zhang's team wrote.

"The aims of this study were to examine possible relationships between endometrial thickness and treatment outcome after IVF/embryo transfer with a relatively large sample size, and to determine what factors affect such relationships," the doctors wrote.

Ready for an Embryo, or Not
For their analysis, Zhang's group collected information on nearly 900 IVF cycles that had been performed in Northwestern University's IVF program between 1999 and 2002. The research team studied information on each patient's endometrial thickness, and determined whether factors like patient age, levels of estradiol (the most significant form of estrogen responsible for many reproductive functions in women), or the number of days of ovarian stimulation had any effect on the thickness of the lining.

In addition to endometrial thickness, the study authors also assessed the impact of patient age, peak estradiol levels, and length of ovarian stimulation on treatment outcome.

Zhang's team found that increasing endometrial thickness and peak levels of estradiol in the blood were associated with improved pregnancy rates, but that older patient age had a negative impact on treatment outcome. The number of days that ovarian stimulation was performed had no influence on therapy outcome, as well.

The investigators also learned that women with higher estradiol levels tended to have thick uterine linings, but the latter had no connection to a patient's age or the number of days of ovarian stimulation.

The Effect of Age
When the investigators specifically examined the impact of a patient's age, thinner endometriums corresponded to lower odds of treatment success only in younger patients. Older women had poorer outcomes regardless of the thickness of their uterine linings. Thinner linings had more of a negative impact in the younger women if ovarian stimulation was performed for a longer length of time, if poorer quality embryos were transferred, or in cases in which embryo transfers were performed sooner rather than later following IVF.

Questions Unanswered
Despite these findings, the researchers could not establish how thick the endometrium must be in order for pregnancy to occur. And since endometrial thickness had no connection to patient age in terms of pregnancy success, this doesn't explain why older patients have lower pregnancy odds in assisted reproduction, Zhang's group wrote.

Further, the study team learned that thin endometriums only have a negative impact on treatment success if embryos are transferred earlier rather than later, and when poor quality embryos are used. Collectively, that suggests that transferring good quality embryos might improve treatment outcome even in women with thin uterine linings, they wrote. Delaying embryo transfer after IVF might also allow the endometrium to develop further, thus increasing the chances of a successful implantation.

"In conclusion, these data suggest that endometrial thickness has predictive value for treatment outcome in younger patients, in patients who require prolonged gonadotropin stimulation, or in those who have poor quality embryos," Zhang's team wrote.

1. Zhang X, Chen CH, Confino E, Barnes R, Milad M, Kazer RR. Increased endometrial thickness is associated with improved treatment outcome for selected patients undergoing in vitro fertilization-embryo transfer. Fertil Steril 2005 Feb;83(2):336-40.

John Martin is a long-time health journalist and an editor for Priority Healthcare. His credits include coverage of health news for the website of Fox Television's The Health Network, and articles for the New York Post and other consumer and trade publications.