Thinness is often associated with fertility problems in couples trying to conceive naturally, often due to underweight women having irregular egg production, irregular periods and producing low levels of the hormone oestrogen.

Obesity can also affect the chances of conception, with excess weight linked to irregular periods, irregular egg production and conditions such as polycystic ovaries.

But while until now most research has focused on the negative impact of obesity, a new study suggests being underweight is just as bad if not more so.

Experts in the US analysed data from 2,362 cycles of IVF involving women under the age of 40.

For women who were underweight - with a body mass index (BMI) of between 14 and 18 - the chance of delivering a healthy baby was 34%.

It was significantly higher (50%) in healthy or slightly overweight women (with a BMI of 19 to 28), and was 45% in very overweight and obese women (BMI of 29 to 43).

Guidelines from the National Institute for Health and Clinical Excellence (Nice) for the NHS say women should ideally have a BMI of 19 to 30 to be accepted for IVF.

However, many primary care trusts (PCTs) set their own weight restrictions on who they will and will not treat.

Dr Richard Sherbahn, from the Advanced Fertility Centre of Chicago, conducted the study and presented his findings at the American Society for Reproductive Medicine (ASRM) conference in Orlando.

He said other research had shown that being too thin was not good for fertility but it was unclear why this should be the case with IVF, when women were given drugs to make them produce eggs.

This should bypass any problems a thin woman may have with period irregularity.

Dr Sherbahn said it was possible, in evolutionary terms, that being too thin may mean the body does not think it is the best time to reproduce.

There may also be differences in the womb's receptiveness to an embryo depending on a woman's weight.

Dr Sherbahn said: "We don't have a really good explanation of the underlying factors.

"The take home message is to have a healthy BMI if possible."

"It's important to look at body weight, having a good diet which is full of fruit and veg and vitamins and minerals, reducing alcohol, stopping smoking and trying to be active."