Around 90 per cent of pregnant women experience changes in their sense of taste – sour or metallic tastes are most common. The effect is strongest in the first trimester, when oestrogen levels are rising fastest, but the exact cause is still unknown. Zinc deficiency has been blamed, but a 2009 study at Tokyo University found that zinc levels were normal in the first trimester.
Another possibility is that the extra hormones affect bladder function. In 2006, researchers at Bristol University found that patients suffering from urine retention problems were more likely to experience odd tastes, and these symptoms vanished when the problem was treated. This may be because the regions of the brain responsible for taste and bladder control are very close together.