The prophylactic treatment of diagnosed preterm delivery with synthetic glucocorticoids (GC), such as dexamethasone (DEX), is commonplace. Long-term effects of such treatment are not well understood. In the present study we exposed pregnant common marmosets (Callithrix jacchus), small-bodied monkeys that are therefore advantageous for long-term primate studies, to daily repeated DEX (5 mg/kg per os) either during early (day 42-48) or late (day 90-96) pregnancy (gestation period of 144 days). Relative to control, we investigated DEX effects in terms of maternal endocrinology (plasma cortisol and oestrogen titres) and offspring physical growth, plasma and urinary ACTH and cortisol titres, and social and maintenance behaviours, from birth to weaning. Both DEX treatments resulted in markedly reduced maternal plasma cortisol titres during treatment and reduced estimated gestation period. Both treatments were without effects on neonate morphometric measurements and basal HPA axis activity. Early DEX treatment resulted in increased infant body weight at postnatal day 56 and 84, co-occurring at the behavioural level with increased time spent in eating solid food, a mobile state, solitary play and exhibiting tail hair piloerection. The constellation of physical and behavioural effects of early DEX suggests interesting parallels with the human metabolic syndrome, providing primate support that the latter is causally associated with the foetal environment, including prenatal programming. This novel primate in vivo evidence for postnatal effects of prenatal GC exposure indicates the importance of improved understanding of this acute clinical treatment in terms its long-term effects on offspring well-being.