Women need to be advised that being too thin can be just as damaging as being too fat when it comes to getting pregnant, according to new research.
Thinness is associated with fertility problems in couples trying to conceive naturally, often due to underweight women having 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 and conditions such as polycystic ovaries.
But while lots of 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 Chicago 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.
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.