Despite widespread anecdotal complaints of impairment, systematic research addressing cognition in gestating women has yielded equivocal results. One way that ambiguous findings could arise is if male and female foetuses affect maternal cognition in different ways. Using a longitudinal within-participants design, we tracked women's cognitive performance from early pregnancy through to postnatal resumption of menstruation. On several cognitive tests, the sex of the foetus was unrelated to maternal performance. But specifically on difficult tests of working memory and spatial ability, a large and enduring effect of foetal sex was evident: women pregnant with boys consistently outperformed women pregnant with girls. A foetal-derived factor that differs in type or concentration between male and female foetuses may thus influence mothers' cognition.