A baby may be small for a variety of reasons, but there are certain overt maternal and fetal factors which may lead to, or are associated with, a poor fetal growth rate. Pre-eminent amongst the maternal factors is severe pre-eclampsia and in women with this disease the smallness of the baby is almost certainly due to the inadequacy of the uteroplacental circulation. Other maternal factors of importance are cigarette smoking, drug abuse and certain infections such as malaria. The most obvious fetal factors associated with a low birth weight are congenital malformations and chromosomal abnormalities, and there the failure of the fetus to achieve a normal weight is clearly an expression of a generalised disorder of growth and is unrelated to the adequacy or otherwise of the placenta. If cases such as these are removed from consideration, there remains an important residue of unduly small infants who are delivered after an apparently uncomplicated pregnancy, are free from congenital malformations and have a normal karotype; it is this group which is considered here.