But how does freezing actually work? Oocyte cryopreservation (OC) was used in animals for many decades, explains Dr Michael Carroll, reader in reproductive science at Manchester Metropolitan University. However, “doing it in humans is relatively new,” he continues. “It was in the late 80s, early 90s, that [scientists] started experimenting on human eggs to see if we could preserve fertility in women.”

Until just over a decade ago, egg freezing was mainly used by women facing premature infertility or undergoing medical treatments like chemotherapy. In 2012, the American Society of Reproductive Medicine declared that it was no longer an experimental procedure. Since then, it has become more widely available around the world for women attempting to preserve their fertility, a process known as “social” or “elective” egg freezing. This is the (potentially lucrative) market that the aforementioned start-ups are honing in on.

“The egg freezing process is very similar to the IVF process, in that it requires injections, anaesthesia and surgery to extract eggs – thus it’s important for women to realise it’s not a walk in the park,” Dr Amit Shah, gynaecologist and co-founder of Fertility Plus, tells Stylist. Patients self-inject hormones to stimulate the ovaries, usually for “between 10 to 14 days”, Shah says. As with IVF, there is a risk of ovarian hyper-stimulation syndrome (OHSS), which can cause fluid to build up in the abdomen or ovaries. According to the RCOG, just over 1 in 100 women will develop moderate or severe OHSS during fertility treatment.

Next, “a fine needle is attached to a scan probe which is passed through the ovary,” Shah explains, and “the fluid in the follicle is aspirated and sent to the embryologist to be checked for an egg.” Vitrification, or “fast freezing”, is now a common preservation method; Carroll says the survival rate of eggs has increased since its introduction, reaching “about 80%”. Eggs are put into a solution of cryoprotectants to “draw the water out”, then plunged into liquid nitrogen and frozen in milliseconds, minimising the chance of ice crystal damage. Still, survival ultimately “depends on the quality of egg, and more importantly, the age of the woman” (the HFEA says it’s best to undergo freezing before 35).

Ettinghausten tells me that while there are currently between 50,000 and 60,000 fertility patients per year in the UK, “there are less than 2,000 who are choosing to freeze their eggs for social reasons”. Despite the relatively small numbers of women undergoing the procedure, elective egg freezing has generated plenty of headlines. Silicon Valley firms started offering it as a perk to female employees in 2014, with Facebook and Apple among the first companies to do so, and this has since caught on in the UK.

“When I worked at a major law firm in London as a junior, the idea of even taking time off work to have a baby – people would gossip about it, like it was outrageous,” says Jenny Wordsworth, the co-founder of fertility wellness brand OVUM, who began her previous career in law in 2009. A decade or so later, Wordsworth says, that same law firm now “offers egg freezing as a benefit”.

It’s not just career goals that might lead a woman to preserve her fertility. Dr Kylie Baldwin is a sociologist at De Montford University, who has researched why women undergo egg freezing. Her interviewees were “unable to pursue motherhood due to the lack of a partner or due to the fact that their partner was unsure about becoming a father… They’d frozen their eggs to try and allow themselves more time to find the right partner to have children with,” she says.

Egg freezing has been given the glossy, millennial pink treatment – but are we being sold a lie?