Successful reproduction has ensured the survival of the human race despite disease and famine. However, a good standard of living, combined with a well-balanced diet, is closely related to low maternal, perinatal and infant mortality rates. In the third world, anaemia, rheumatic heart disease, tuberculosis and other infections are still common and the poorly nourished are more susceptible to them. In the Western world food is plentiful but often the individual's diet is poorly balanced. Invariably the fetus takes all the nutrients required from the mother and, if a deficiency occurs, this is reflected in the maternal reserves only. The weight of the baby at birth has a wide biological variation and correlates in the extremes with maternal weight but there are many other factors responsible for fetal growth. In early pregnancy the developing embryo may be susceptible to specific dietary deficiencies and structural malformations may result. A clear link has recently been established between folic acid supplementation and the prevention of recurrence of neural tube defects. Excessive doses of vitamin A should be avoided.