The dietary requirements for a healthy pregnancy begin before conception. Women who are underweight find it difficult to conceive, as do those who are overweight, but the latter are more likely to suffer from complications during pregnancy.

Any woman contemplating pregnancy should maintain a body mass index (BMI) between 20 and 25.

Taking at least five portions of fruit and vegetables each day not only helps to maintain weight and overall health, but contributes to the supply of folic acid.

Women anticipating pregnancy should take daily supplements (400 micrograms) of folic acid. It is essential both when trying to get pregnant and during the first 12 weeks of pregnancy.

Most diets are not low in vitamin A, and should not be supplemented during pregnancy, as this can lead to birth defects.

Embarking on a new dietary regime immediately before or during pregnancy can jeopardise nutrient supply and is not recommended.

Although there is an increased requirement for protein, energy and other nutrients during the first six months of pregnancy, the woman's body becomes super-efficient and normally there is no need to "eat for two". Of course, if normal nutrient needs are not met before pregnancy - or if there is an extra demand for nutrition (such as happens in teenage pregnancy) - then nutritional risk increases.

The growing baby takes priority on the mother's available nutrients, so it is likely to be the mother who suffers most from any dietary inadequacy.

Smoking, iron deficiency anaemia and high levels of exercise and caffeine can all contribute to a reduced supply of nutrients to the baby and result in a low birthweight - which in turn appears to raise the risk of heart disease and hypertension in adulthood.

· Anne de Looy is professor of dietetics at the University of Plymouth