Angelina Jolie, it is reported in a women's magazine, wants another child with Brad Pitt, but has lost too much weight to fall pregnant. When it comes to making babies, you can indeed be too thin. Jolie has reportedly lost weight not intentionally, but due to stress following the death of her mother. Whatever the reason, one of the first signs of severe weight loss (as seen in anorexics) is that the hypothalamus in the brain tells the body not to ovulate, and periods stop. If you aren't taking in enough calories for yourself, Mother Nature clearly believes you aren't eating enough to grow a baby.

Dr Diane Lockwood, medical director of Midlands Fertility Services, says babies are not such perfect parasites that they can extract everything they need from the slimmest of maternal pickings. "Women who are very thin have higher risks of miscarriage and babies born prematurely," she says. "There is overwhelming evidence that thin women have underweight babies."

The degree of thinness we are talking about is a body mass index (body weight in kilograms over height in metres squared) of less than 19. Guidelines from the National Institute of Clinical Excellence suggest that women shouldn't try infertility treatment until they are within a BMI range of 19-30 because being both under- and very overweight makes it less likely to work. Women with a BMI under 19 are usually as insubstantial as the size-zeros on catwalks (5ft 9in and 9st 9lb gives a BMI of 18.6).

When thin women return to more normal weights, their periods don't always resume, and then they may need drugs to give them a kick-start. Some women really do have a high metabolic rate and a BMI of under 19 despite eating ferociously, in which case their fertility should be fine. But that just shows that life is deeply unfair.