Stem cell embryo models could unlock infertility clues beyond 14-day research limit

by Universitat Pompeu Fabra - Barcelona

edited by Gaby Clark, reviewed by Robert Egan

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An international team of experts in embryology and bioethics has published the first white paper on the use of embryonic models based on stem cells in the field of reproductive biology. The document, coordinated by UPF researcher Alfonso Martínez-Arias, offers an integrated view of developmental biology, bioethics and legislation, and proposes guidelines for the correct implementation of these models in research and in the regulation and development of technologies related to human reproduction. This text should facilitate the use of models to study embryo development beyond the 14-day legal barrier, hopefully leading to an understanding of some of the conditions that cause infertility.

It is estimated that 1 in 6 people of reproductive age experiences fertility problems at some point in life. Worldwide, infertility affects 48 million couples. Although assisted reproduction techniques have enabled the birth of more than 10 million babies worldwide over the past 30 years, much remains to be understood about why, in humans, only 1 in 3 fertilized eggs progresses beyond the third week of development.

"We are very knowledgeable about what happens during the first seven days of embryonic development; we know much less about what happens during the following week, in which the embryo becomes implanted in the uterus, and, for ethical and legal reasons, we cannot take our research further." In particular, exploring the third week is crucial in our study, explains Alfonso Martínez-Arias, an ICREA researcher, professor emeritus at Pompeu Fabra University in Barcelona and project leader.

"This limitation prevents us from seeing how, at the end of the third week, the embryo has transformed, is organized and primed for organogenesis." According to Martínez-Arias, during this process, known as gastrulation, "many of the cardiovascular and metabolic disorders or even limb malformations that are detected after birth originate."

A model for studying the early stages of human development

The so-called "14-day rule" prohibits the cultivation of human embryos in the laboratory beyond this period. In addition, the embryos used for research are surplus ones donated by assisted reproduction clinics. The supply is limited and subject to ethical restrictions that do not allow the investigation of gastrulation.

Embryonic models based on stem cells—that is, copies of embryos generated in the laboratory from cells that can become any cell in the body—are, conversely, a good alternative. Although in humans it has not yet been possible to go beyond the early stages of gastrulation, in monkeys they can already be used to mimic it.

To answer the biological, ethical and legal questions involved in the use of these models in research and in the regulation and development of technologies related to human reproduction, a white paper has been published in Human Reproduction. Coordinated by biologist Martínez-Arias, the work includes the opinion of experts from different fields who, during a meeting at the Institut Pasteur in Paris in November 2024, discussed the requirements for using these models properly.

"If we want embryonic models based on stem cells to be useful for research and the treatment of infertility, we must have reliable, reproducible models with a legal framework that allows us to use them," the development expert states.

In the document, the experts advocate the need to standardize the models at the highest level so that they can be used for clinical practice and the development of devices that can improve assisted reproduction techniques. "The disparity of methodologies and cell lines used to create the models means that the results are not yet comparable to human embryos." Nevertheless, the experts hope to break this barrier in the not-too-distant future.

In their opinion, embryos and models should not be subject to the same regulations. "We agree that an embryo should not be cultured beyond 14 days, but it is important to explain the value of stem cell models as an extraordinary resource to understand this period, when many human diseases originate, as a replacement for embryos," Martínez-Arias adds.

Precisely because they are not the same, the white paper makes the red lines clear: The models must never be transferred to the uterus of any animal or human and must be cultivated only for the time necessary for each study. These limits, the experts suggest, must always be overseen by ethics committees and must be communicated responsibly and transparently.

Publication details

Alfonso Martinez Arias et al, Human stem cell-based embryo models: innovation, ethics, and policy, Human Reproduction (2026). DOI: 10.1093/humrep/deag035

Journal information: Human Reproduction

Key medical concepts

InfertilityGastrulationAssisted Reproductive Technology

Clinical categories

Obstetrics & gynecologyReproductive health Provided by Universitat Pompeu Fabra - Barcelona Who's behind this story?

Gaby Clark

MA in English, copy editor since 2021 with experience in higher education and health content. Dedicated to trustworthy science news. Full profile →

Robert Egan

Bachelor's in mathematical biology, Master's in creative writing. Well-traveled with unique perspectives on science and language. Full profile →

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