Even in the best fertility clinics, transferring a healthy embryo into the uterus leads to a live birth only about half the time. In 30% to 35% of cases, the embryo does not implant at all. Other times, implantation happens but the pregnancy ends in miscarriage.

For years, most fertility research has focused on the embryo. But new research from Michigan State University and Rutgers Health suggests that the health of the uterus itself may be a key part of the puzzle.

The study, recently published in JCI Insight, identifies genes in the uterine lining, or endometrium, that activate in fertile women when the uterus enters its brief window of receptivity. The research was supported by three grants from the National Institutes of Health, including the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

“Getting pregnant is not just about the egg and sperm,” said Greg Burns, one of the lead authors on the research and assistant professor in the MSU College of Human Medicine Department of Obstetrics, Gynecology and Reproductive Biology. “The lining of the uterus, or the endometrium, must be ready to welcome an embryo. Out of about 20,000 possible genes, we identified 556 that work together like a health checklist for a normal uterus. For women who have trouble getting pregnant, this gene pattern often looks different.”

Understanding this group of genes, known as the Glandular Epithelium Receptivity Module, or GERM, could lead to better tests and treatment for infertility.

“Both the ‘soil’ and the ‘seed’ have to be healthy,” said Ripla Arora, associate professor in the MSU College of Human Medicine’s Department of Obstetrics, Gynecology and Reproductive Biology.

To learn what makes a uterus truly ready to host an embryo, researchers studied endometrial tissue from 30 healthy women with proven fertility and regular cycles. This study included Black and Hispanic subjects. The data obtained was also compared against other published studies that included Asian and white women and showed that the same basic biological patterns appear across racial and ethnic groups.

The most striking changes in the endometrium were found in specialized cells within uterine glands that produce molecules thought to support implantation and nourish an embryo. This is the first human evidence for the uterine glands’ central role.

“Fertility is not controlled by just one gene,” said Arora, who co-authored the research. “It’s a very complex process. You need many cell-to-cell signals rising and falling together at the right time. Our study shows that successful implantation depends on that balance — even if the embryo is healthy.”

When the researchers applied the GERM pattern to nearly 190 patients with infertility, the signal was consistently weaker in women with recurrent implantation failure or pregnancy loss. Previous gene-based tests have looked at smaller groups of genes, but results have been mixed, and recent clinical trials have not consistently improved pregnancy rates.

Although the research is still in early stages, the potential patient benefits are significant. In the future, doctors may be able to more accurately test whether the uterus is truly ready before embryo transfer. This would allow them to improve invitro fertilization timing, provide clearer answers for patients with repeated implantation failure, and develop treatments that correct specific uterine problems.

“Our hope is that this work moves us closer to identifying patients whose uterus isn’t fully prepared for pregnancy and finding ways to help,” Arora said. “Understanding uterine readiness is essential if we want to improve pregnancy success.”

Additional authors include Emmanuel Paul from MSU and Anat Chemerinski and Nataki Douglas from Rutgers University.

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