For years, many fertility specialists have viewed a thin endometrial lining—the inner uterine layer that supports embryo implantation—as a major barrier to successful in vitro fertilization (IVF). In the U.S., patients with a thin endometrium often face delayed IVF treatments until the lining thickens sufficiently.
However, recent research published in Human Reproduction challenges this long-held belief, suggesting that a thin endometrium may not drastically reduce the chances of a successful pregnancy.
The international study analyzed over 30,000 IVF cycles from 25 fertility centers across the U.S., Spain, and the United Arab Emirates (UAE). It focused on frozen embryo transfers of single, genetically tested embryos and compared outcomes based on endometrial thickness, defining thin as less than seven millimeters.
In clinics in the U.S. and Spain, where doctors commonly postpone embryo transfer in cases of thin endometria, only 3% to 5% of transfers occurred with thin linings. Contrastingly, UAE clinics, which are less cautious about endometrial thickness, conducted approximately 12% of transfers with thin linings.
Crucially, the study found no significant reduction in live birth rates in UAE patients with thin endometria. Conversely, clinics in the U.S. and Spain reported a 20% decrease in live births among patients with endometrial thickness below seven millimeters.
Lead investigator Dr. Emre Seli of Yale School of Medicine explains this discrepancy is likely due to patient selection differences. In the U.S. and Spain, patients with thin linings often receive medical interventions or have cycles canceled if the lining fails to thicken, meaning those who proceed tend to be a select group resistant to treatment. This resistance, rather than the thin lining itself, may underlie the lower birth rates observed.
“It’s probably not the thin lining causing the issue but an underlying factor affecting both the lining thickness and implantation success,” Dr. Seli states. “Many patients with thin linings still have favorable pregnancy outcomes.”
The study also highlights successful pregnancies occurring even with endometrial thicknesses below five millimeters, demonstrating that pregnancy is possible at thicknesses as low as four to six millimeters, albeit with a modestly reduced success rate.
Looking ahead, Dr. Seli’s team aims to develop predictive tools to help fertility specialists better counsel patients on their chances of pregnancy, regardless of endometrial thickness.
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