Fertility experts have highlighted a worrying reason why ultra-trim celebrities such as actress Lily Collins opt for surrogate births — they struggle to conceive due to their lack of body fat.

Specialists say that many slender celebs are unlikely to have the minimum level needed for regular ovulation, when an egg is released for fertilisation.

Speaking to MailOnline, Renee McGregor, a dietician specialising in eating disorders, said natural conception with such a super-slim frame would be ‘almost impossible’.

‘Research shows that ovulation occurs only when body fat ratio is above about 21 per cent,’ she said. ‘Generally speaking, if you can see a woman’s abdominal muscles, their body fat percentage is hovering at about 18 per cent.

‘Women won’t be having periods or ovulating with less than 20 per cent body fat – let alone be able to conceive.’

It comes amid rife speculation as to the reason Ms Collins opted to have her daughter Tove via a surrogate.

The Emily In Paris star, 35, announced the birth of her first child with film director husband Charlie McDowell last week, expressing ‘endless gratitude’ for the surrogate who helped her become a mother.

But social media users have attacked the couple’s choice, accusing them of taking part in the ‘unethical trend’ of ‘renting women’s bodies’.

In an emotional post on Instagram, the Emily In Paris star said she was full of ‘endless gratitude’ for the surrogate who helped her to become a mother

Some speculated that surrogacy is often chosen by A-list celebrities as a way of avoiding pregnancy to preserve their looks.

Los Angeles-based fertility doctor Vicken Sahakian has echoed this theory, arguing that ‘social surrogacy‘ allows some women to continue working without having to pause their careers.

She said: ‘For instance, if you are a model or an actress and your income is based on performing and how you look, pregnancy will ruin that for a year if not more.’

But other experts pose an entirely different theory.

Ms McGregor says it’s likely the extreme diet and exercise regimes adopted by famous women plays havoc with their reproductive system.

‘You don’t have to starve for the body to be under-fuelled. Many of my patients [who don’t have periods] have adopted restrictive diets they’ve seen on social media,’ she explained.

‘Others are doing trendy fasting diets while following high-intensity workouts.

Experts say many celebrities are too slim to conceive naturally, as periods stop if women don’t have a certain threshold of body fat. Pictured: Lily Collins in October 2024

 ‘Often the body needs more weight than you might think it needs, following a period of under-nourishment, to “decide” it’s safe to carry a baby.

‘A certain amount of body fat is needed, as well as enough carbohydrate. It’s only when there’s enough fat and carbohydrate that signals are sent to the brain, firing up the reproductive system.’

Ms Collins, who has spoken openly about her teenage struggles with eating disorders, is one of several slim celebrities who have opted to have children via a surrogate.

Hollywood star Nicole Kidman, who has been married to country music legend Keith Urban since 2006, had her fourth child, Faith Margaret Kidman Urban, via surrogate in 2011, aged 43.

The actress, now 57, has previously revealed that during her 11-year marriage to Tom Cruise she suffered an ectopic pregnancy and a miscarriage. They adopted two children — Bella, now 32, and Connor, now 30.

The surrogate birth came three years after she welcomed her first biological child, aged 40, following years of fertility struggles.