In 1996, British man Peter Hickles was diagnosed with Hodgkin lymphoma at only 21 years of age. This is a very rare type of cancer that affects the lymphatic system, which is part of the body’s immune system. But Hodkin lymphoma is one of the most treatable cancers and Hickles was eventually cured. However, the treatment cost the man’s fertility.

Aware of the risks, Hickles froze his sperm before commencing chemotherapy. He was informed that the sperm would last no more than ten years, but last week the man become a father using sperm he had kept frozen for more than 26 years. It’s almost a record for the longest gap between sperm collection and birth, only a year away from a 27-year-old sample from the United States.

“I keep looking at him, shaking my head in disbelief. He really is a little miracle. The fact that he was basically ready to go just before Euro 96 and was born before the World Cup is amazing,” Hickles said in a statement.

Hickles was very lucky that his condition was exceptional. Until this summer, frozen human sperm and eggs could only be stored for no more than 10 years in the UK, with exceptions made for patients with fertility problems such as Hickles. But since then, a new law has been passed that has extended the legal limit to 55 years.

Biologically speaking, however, babies could theoretically be born from frozen sperm even hundreds of years after their fathers had passed — there’s no scientific reason why this couldn’t be possible.

“The legal 55-year limit has nothing to do with the shelf life of sperm, or for any other scientific reasons. It’s more to do with what parliamentarians felt was right for society. But since frozen sperm are effectively in suspended animation, once they are frozen I don’t see why they couldn’t be kept for hundreds of years if the law allowed it,” Allan Pacey, a professor of andrology at the University of Sheffield, told The Guardian.

Once sperm is collected and prepared, fertility clinics place it in special vials with the donor’s label and store the sample in large tanks where sperm is kept very, very cold by liquid nitrogen at -196 degrees Celsius. Each patient’s sperm is typically split into several vials and kept in multiple storage tanks. If one freezer fails, the remaining sperm samples remain safe.

All signs point to the fact that sperm can be kept frozen indefinitely. The world’s oldest known viable semen collected in 1968 was used to impregnate 34 Merino sheep. In 2019, researchers reported that the resulting live birth rate was as high as artificial insemination using sperm frozen for just 12 months.

“This demonstrates the clear viability of long-term frozen storage of semen. The results show that fertility is maintained despite 50 years of frozen storage in liquid nitrogen,” said Associate Professor Simon de Graaf from the Sydney Institute of Agriculture and School of Life and Environmental Sciences at the University of Sydney.

Although studies on the decades-long viability of frozen human sperm are lacking, there are good reasons to believe frozen human sperm could last at least just as long as that from rams or bulls. One recent study that examined the clinical outcomes of nearly 120,000 semen specimens at sperm banks in China — with some of these samples stored for up to 15 years — found no differences in the rate of pregnancies, miscarriages, and live births between sperm of different storage times.

That doesn’t mean age doesn’t matter at all. The same study found that the longer sperm is frozen, the lower the survival rate of the sperm once it is thawed. So a sample that’s stayed frozen for 15 or 20 years will have a lower percentage of functional sperm compared to a sample frozen for only 12 months. But because a sample contains millions of individual sperm cells, the overall fertility rate doesn’t seem to be affected, although all of this suggests there may be a hard ceiling for how long human sperm can be stored viably. This ceiling however could be centuries or even thousands of years away from the date of collection, at which point it makes sense to call it ‘indefinite’ storage.

So, theoretically, new babies could be born centuries after their fathers lived, which raises some interesting possibilities but also ethical challenges. Very famous and powerful people could make it such that they sire direct descendants for many generations to come. For instance, imagine a very wealthy individual who funnels hundreds of millions into a personal fertility fund, enough to ensure that every 25 years or so his sperm is used to inseminate a volunteer who agrees to raise the child. Perhaps some mothers would even seek this service out for free if the donor is someone extremely famous or known for their ‘exceptional genes’. Imagine direct descendants of Albert Einstein living in the year 2734. That’d be pretty wild and not necessarily so far-fetched.