Another routine screening practiced in certain countries is for so-called gestational diabetes. This is the reason for using the glucose tolerance test. If the glycaemia (amount of glucose in the blood) is considered too high after absorption of sugar, the test is positive. This diagnosis is useless because it merely leads to simple recommendations that should be given to all pregnant women. These are recommendations regarding lifestyle, particularly dietary habits and exercise. Dietary recommendations must focus on the quality of carbohydrates. The most useful way to rank foods is according to their ‘glycaemic index’. Pregnant women must be encouraged to prefer, as far as possible, low glycaemic index foods. A food has a high index when its absorption is followed by a fast and significant increase of the blood glucose level. In practice this means, for example, that pregnant women must avoid the countless soft drinks that are widely available today, and that they must also avoid adding too much sugar or honey in their tea or coffee. Glycaemic index tables of hundreds of foods have been published in authoritative medical journals. These tables must be looked at carefully, because the data they provide are often surprising for those who are still influenced by old classifications contrasting simple sugars and complex carbohydrates. Such classifications were taking account the mere chemical formula.
From such tables we can learn in particular that breakfast cereals based on oats and barley have a low index. Wholemeal bread and pasta also are low-index foods. Potatoes and pizzas, on the hand, have a high index and should therefore be consumed with moderation. Comparing glucose and fructose (the sugar of fruit) is a way to realize the lack of correlation between chemical formula and glycaemic index. Both are small molecules with six atoms of carbon and have pretty similar chemical formulas. Yet the index of glucose is 100…versus 23 for fructose. This means that pregnant women must be encouraged to eat fruit and vegetables.
The benefits of a regular physical activity in pregnancy should also be a routine discussion during prenatal visits, whatever the results of sophisticated tests. A huge Canadian study demonstrated that the only effect of routine glucose tolerance screening was to inform about three per cent of pregnant women that they have gestational diabetes (20). The diagnosis did not change the birth outcomes.