In mammals, allocation to reproduction can either be primed or suppressed in relation to cues from other individuals. The presence, odour and behaviour of conspecifics (e.g. potential mates) may enhance an individual’s ability to reproduce but others may have a detrimental effect on reproductive success. One widely studied response to conspecific cues, the ‘Bruce effect’, occurs when pregnant females abort their pregnancies after exposure to a novel male. It has been suggested that this response has evolved as a counter-tactic to the threat of infanticide posed by novel males. In some species, like mice, pregnancy termination will only occur if females are exposed to the unfamiliar male during a brief critical period early in pregnancy, which is surprising considering that an unfamiliar male threatens infanticide whenever present, and in particular near to birth. We demonstrate that female mice experiencing novel males during late pregnancy also alter their investment in progeny. These females give birth to offspring of a comparable weight to those produced by females exposed to the paternal male, but their offspring grow more slowly over lactation. As a consequence, offspring from these females weigh less at weaning. Modification of their growth trajectory, however, allows these offspring to catch up to normal weights by adulthood. Further, we also explore whether the novel male presented in late pregnancy elicits a stress response in the pregnant females and if there are further associated costs to the offspring’s behaviour or physiological and reproductive fitness.