According to a Dutch study of 500 women, the fatter a woman's waist is relative to her hip size, the longer it takes her to get pregnant. Whilst doctors have long known that women who are too thin or obese have more difficulties getting pregnant than women of average weight have, this finding suggests that fertility is not only affected by weight, but by how the weight is distributed. Even for normal-weight women, those of an apple-shape have a lower probability of conception that pear-shaped women. The distribution of fat may act like a chemical signal which triggers higher levels of female hormone in pear-shaped women.