First come the egg and sperm. Then, for a healthy pregnancy, comes … bone marrow? Such is the conclusion of new fertility research from Yale University, which, by studying mice, determined that once an egg is fertilized, stem cells from a woman’s bone marrow travel via the bloodstream to prepare the uterine lining for implantation.

“… knowing that bone marrow has a significant role [in pregnancy] is a paradigm shift,” Dr. Hugh Taylor, professor of obstetrics, gynecology, and reproductive sciences at Yale, said in a statement. Dr. Taylor is the senior author of the study published recently in the journal PLOS Biology.

For several years, fertility research focused on cracking what endometrial receptivity, a window of time when the endometrium — that is, the uterine lining — is supportive of a fertilized egg implanting in it. Endometrial receptivity is important not only for the success of so-called natural conception but also for success during the transfer of fertilized eggs in assisted reproductive processes. Previous research has suggested the involvement of stem cells from bone marrow in sustaining the lining of a non-pregnant uterus, but whether they are a factor in egg implantation and endometrial receptivity post egg fertilization has been unknown — until now.

“Some of these bone marrow-derived mesenchymal stem cells travel to the uterus and become decidual cells, which are the cells that are essential for the process of implantation and pregnancy maintenance,” Dr. Reshef Tal, first author of the study and an assistant professor of obstetrics, gynecology and reproductive sciences at Yale, said in the statement.

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Researchers genetically engineered mice in two ways to present with poor endometrial receptivity similar to humans. In the first experiment, in which mice had only one copy of a faulty gene, a bone marrow transplant saved pregnancies that would have otherwise been lost and increased litter size. In the second experiment, in which mice had two copies of the faulty gene — which manifests as complete infertility — a bone marrow transplant resulted in the growth of healthier endometrial tissue.

It’s worth noting the discovery was made on mice, which means while the finding can and should inform additional research, perhaps involving humans in the future, it is a far cry from being the basis of any human-applicable treatment yet. Still, the researchers say their finding suggests a day in the future when bone marrow-related treatments could be an additional avenue for addressing fertility problems in human patients.

“These are frustrating medical conditions,” Dr. Taylor said, referring to the endometrial conditions the researchers induced in the mice. “When you have a damaged endometrium leading to infertility or repeated pregnancy loss, all too frequently we have not been able to correct it. Bone marrow can be considered another critical reproductive organ. This finding opens up a new potential avenue for treatment of a condition that has been untreatable in the past.”