Speaking to the Guardian, Prof Katsuhiko Hayashi, a developmental geneticist at the University of Osaka, said rapid progress is being made towards being able to transform adult skin or blood cells into eggs and sperm, a feat of genetic conjury known as in-vitro gametogenesis (IVG).

His own lab is about seven years away from the milestone, he predicts. Other frontrunners include a team at the University of Kyoto and a California-based startup, Conception Biosciences, whose Silicon Valley backers include the OpenAI founder, Sam Altman and whose CEO told the Guardian that growing eggs in the lab "might be the best tool we have to reverse population decline" and could pave the way for human gene editing.

"I feel a bit of pressure. It feels like being in a race," said Hayashi, speaking before his talk at the European Society of Human Reproduction and Embryology's (ESHRE) annual meeting in Paris this week. "On the other hand, I always try to persuade myself to keep to a scientific sense of value."

If shown to be safe, IVG could pave the way for anyone – regardless of fertility or age – to have biological children. And given that Hayashi's lab previously created mice with two biological fathers, theoretically this could extend to same-sex couples.

"We get emails from [fertility] patients, maybe once a week," said Hayashi. "Some people say": 'I can come to Japan.' So I feel the demand from people."

Matt Krisiloff, Conception's CEO, told the Guardian that lab-grown eggs "could be massive in the future".

"Just the aspect alone of pushing the fertility clock … to potentially allow women to have children at a much older age would be huge," he said. "Outside of social policy, in the long term this technology might be the best tool we have to reverse population decline dynamics due to its potential to significantly expand that family planning window."