A food's 'glycaemic index' (GI) is the degree to which it causes a surge (called a 'spike') in blood sugar levels. It is an important measure because spikes in blood sugar:

• trigger the pancreas to release large amounts of insulin into the body (which makes people more susceptible to a variety of illnesses, including diabetes, birth defects, heart disease and cancers)

• are always followed by blood sugar 'plummets' to way below adequate levels

Typical high GI foods are easily-digested carbohydrates like sugar, potato, white bread, white rice and many breakfast cereals.

Gary Shaw and colleagues from the University of California Birth Defects Monitoring Programme analysed the diets whilst pregnant of nearly 1,000 women. When they compared those of women who had given birth to a healthy baby with those whose baby had been born with neural tube defects (the cause of, for instance, spina bifida), they found that those who had eaten a high GI diet while pregnant appeared to have doubled their risk of a baby with birth defects. In obese high-GI mothers there was a fourfold risk (Shaw,GM. American Journal of Clinical Nutrition 2003;78:972-77).

Tom Wolever, chair of nutritional sciences at the University of Toronto, cautioned mothers against overreacting. "This kind of (birth) defect is very unusual. (A high GI diet may have) doubled the risk, but the risk is very, very low. Double a low number is still a very low risk."