Sheep born from eggs matured entirely outside the body are the first large animal evidence that an IVF approach aimed at patients with damaged or depleted ovaries could one day work in humans.
Researchers at the University of Leeds have produced live lambs from eggs that were matured entirely in the lab from highly immature ovarian follicles, the first live births in a large animal for a technique called in vitro growth, or IVG. The work, presented this week at the annual meeting of the European Society of Human Reproduction and Embryology (ESHRE) in London, points toward giving fertility patients with few or no mature eggs a new path to treatment.
The method works on a supply a female is born with. A human ovary holds tens of thousands of primordial follicles, each containing an immature oocyte, that never get the hormonal signal to finish maturing. Standard IVF tries to rescue a fraction of them with one to two weeks of daily hormone injections, typically retrieving around ten mature eggs per cycle and fertilizing perhaps six to eight, and still ends in a live birth only about one cycle in five. The Leeds group took a different route: collect the immature follicles directly, bathe them in hormones and growth factors in the lab, and grow the eggs inside them to maturity. The follicles are taken from the ovary, not generated from stem cells, which sets the work apart from a parallel line of egg-from-stem-cell research that has produced human egg precursors in the lab but no human pregnancies.
According to the New Scientist report on the work, roughly 60% of the immature sheep follicles developed into eggs usable for fertilization, and about 30% of those eggs fertilized successfully before being transferred into surrogate ewes. The team reported healthy lambs, including a female born in early 2024 that has since had two lambs of her own, a check that the lab-matured eggs are functionally normal, not just structurally complete.
IVG was first demonstrated in mice more than 30 years ago, but until now no large animal had produced live young from lab-matured eggs. The healthy 2024 lamb and her own two offspring are the first such evidence in a species whose reproductive biology is closer to a human's. Sheep reach puberty, gestate for months, and produce one or two offspring at a time, which makes them a more demanding test than mice, which litter a dozen pups every few weeks. Two independent fertility specialists quoted in the same New Scientist report, Stine Kristensen of Copenhagen University Hospital and Manjushree Boob of Shubham Hi-Tech Hospital and Test Tube Baby Centre in Maharashtra, India, both treated the result as a meaningful step toward human application, with Boob noting the closer resemblance of sheep reproduction to human reproduction.
The clinical use case the Leeds group is working toward is the patient who has very few or no mature eggs to retrieve in the first place: women whose ovaries have been damaged by chemotherapy or radiation for cancer, or who have a diminished ovarian reserve for other reasons. For them, the appeal of IVG is that it draws on the body's existing stockpile of dormant follicles rather than the much smaller pool that a hormonal cycle can recruit.
The result does not change IVF outcomes today. The work is conference-stage data, not a published paper, the number of live births is small, and the standard IVF live-birth rate of roughly one in five per started cycle is the benchmark the approach has not yet moved. No human embryos have been made this way, and the team has not given a timeline. The next test will be in human tissue, and the first signal that IVG has a clinical future will be a credible result there.
The University of Leeds profile of Helen Picton, the reproductive biologist leading the work, frames the research as a long-running effort to give clinicians a way to recover usable eggs when none are available to harvest. The ESHRE 2026 annual meeting in London, where the sheep data was presented, runs through 8 July.