Half a million pregnant women are troubled by morning sickness each year in the UK, but this phenomenon is still not fully understood. One theory is that changing levels of hormones act on the brain to heighten an evolutionary adaptive response that helps prevent women from consuming substances that may be harmful. Although less important in the developed world, in the past this would have helped to protect a foetus during the first three months of pregnancy when it is at the most vulnerable stage of development. If the mother eats less, and sticks to simple foods, she is less likely to accidentally ingest something dangerous.