Being too thin is worse than being too fat when trying for a baby, women have been warned.
A study found that skinny women are less likely to become pregnant than those who are overweight – including those classed as dangerously obese.
The researcher say that the amount of attention being paid to the health risks of being overweight meant that the perils of being underweight are being largely ignored.
The problem is being exacerbated by the ‘size zero’ culture in girls and young women striving to emulate the painfully thin look of models and other celebrities.
Dr Sherbahn, of the Advanced Fertility Center of Chicago, crunched the figures on almost 2,500 sessions of IVF carried out at his clinic over an eight-year period.
The women were divided into three groups by weight – very thin, normal and obese. The normal weight group included some women who would be classed as overweight in the UK.
Some 50 per cent of those in the normal weight group had babies.
This compared with 45 per cent of those in the obese group, which included women classed as dangerously obese, and just 34 per cent of those classed as very thin.
The women classed as very thin had a BMI, or body mass index of 14 to 18.
A woman who is 5ft 4in tall and weighs 7stone will have a BMI of 17. One who is 5ft 10ins tall and weighs 9 stone will have a BMI of 18.
Dr Sherbahn said that while some other studies had hinted that being skinny may be worse for fertility than being fat, he was ‘surprised’ at the size of the effect.
It is known that being very thin can make it difficult to get pregnant naturally, due to a drop in the female sex hormone oestrogen.
But women undergoing IVF are given hormones, so this couldn’t be the reason for the results.
The women in all three groups produced similar numbers of eggs, so the problems for the very thin later must have occurred at a later stage in the process, the American Society for Reproductive Medicine’s annual conference heard.
One possibility is that the embryos found it more difficult to implant in the wombs of the very thin women because they were undernourished.
Dr Sherbahn said: ‘It could be in evolutionary terms that if people were too thin that maybe food wasn’t readily available and maybe it wasn’t the best time to reproduce and maybe the uterus wasn’t at its best.’
He added that women are likely unaware that it can be more damaging for their fertility to be too thin rather than too fat.
‘I am no expert on the sociological side of it but I have a teenage daughter and it seems that girls idolise models who are anorexic-looking.
‘It seems that the ideal body structure for young women is this overly-skinny physique and women don’t understand that there is any concern about that.’
He said that women trying to get pregnant – naturally or with fertility treatment – should try to get as close to their ideal eight as possible.
Charles Kingsland, a consultant gynaecologist at the Liverpool Women’s Hospital and member of the British Fertility Society, said: ‘For some people, getting pregnant is very easy but for others it is difficult and it is important to look at your bodyweight.
‘There is no doubt that if it is appropriate for your height, you have a higher chance of conceiving.’
WHY OESTROGEN COULD BE USED IN DIETING
Lack of oestrogen can increase appetite and slow down metabolism – paving the way for the hormone to be used as a diet aid.
Researchers in the U.S. discovered that low levels of the female sex hormone in certain parts of a woman’s brain can lead to obesity.
The findings, from tests in mice, are at an early stage, but could have consequences for millions of post-menopausal women if the link can be proved in humans in future research.
Dr Deborah Clegg, who led the study, said: ‘Oestrogen has a profound effect on metabolism. We hadn’t previously thought of sex hormones as being critical regulators of food intake and body weight.’
Oestrogen receptors are located throughout the body, but the researchers found two specific places in the brain where they appear to regulate energy balance.
Female mice whose brains lacked oestrogen in these parts became obese and developed illnesses such as diabetes and heart disease.