A suboptimal in utero environment, for example, as a result of maternal stress, can have detrimental effects on the pregnancy and long-term adverse 'programming' effects on the offspring. This article focuses on the effects of prenatal social stress on the mother, her pregnancy and the offspring, since these issues have ethological relevance in both animals and humans. The consequences of social stress exposure depend on when during pregnancy the stress occurs, and many of the effects on the offspring are sex specific. Social stress during early pregnancy tends to result in pregnancy loss, whereas stress exposure later in pregnancy, when the mother has already invested considerable resources in the foetuses, results in programmed offspring of low birth weight: a risk factor for various adulthood diseases. Neuroendocrine and behavioural responses to stress in the offspring are particularly sensitive to foetal programming by prenatal stress, indicated by enhanced hypothalamo-pituitary-adrenal (HPA) axis responses and increased anxiety behaviour, which result from permanent changes in the offspring's brain. The dysregulation of HPA axis function may also interfere with other systems, for example, the hypothalamic-pituitary-gonadal axis, as there is evidence for alterations in steroidogenesis, reproductive potential and impaired reproductive/social behaviours in prenatally stressed offspring. Prenatal social stress also programmes future maternal behaviour, highlighting the potential for negative phenotypes to be transmitted to future generations. The possible mechanisms through which maternal stress during pregnancy is transmitted to the foetuses and the foetal brain is programmed by prenatal stress and the potential to overwrite programming of the offspring are discussed.