This article reviews different aspects of maternal weight before and during pregnancy and weight gain in pregnancy, e.g. causes of undernourishment (hyperemesis, anorexia nervosa and bulimia nervosa). Physiological weight gain during pregnancy is normally between 10 and 16 kg, representing 20% of the body weight before pregnancy. The increase in weight is usually lowest during the 1st trimester and greatest between the 17th and the 24th week of pregnancy. Low maternal weight at conception may cause low birthweight. Undernourishment may cause premature delivery or low birthweight, or both. There is an increased risk of gestational diabetes and macrosomia, as well as preterm delivery and hypertension in pregnant women who are overweight. There is also an increased risk of complications arising during general anaesthesia and operative delivery in severely overweight women. These women should be offered heparin or dextran as thrombosis prophylaxis where a caesarean section is to be performed. They should also be given antibiotic prophylaxis. A weight gain of between 7 and 12 kg reduces the risk of complications in overweight patients.