BACKGROUND: Anecdotal reports of sleep disturbance during pregnancy are abundant; however, objective measurement of sleep changes has so far produced conflicting results. AIMS: To objectively measure sleep architecture and investigate subjective sleep quality in the first and third trimester of pregnancy, when compared to the nonpregnant state. METHODS: Twenty-seven women in the third trimester of pregnancy, 21 women in the first trimester of pregnancy and 24 nonpregnant control women underwent overnight polysomnography and completed questionnaires regarding sleep quality and mood. RESULTS: Women in the third trimester of pregnancy had poorer sleep efficiency, more awakenings, less stage 4 sleep, more stage 1 sleep and fewer minutes in rapid eye movement sleep when compared to the control group. Cortical arousals were seen more often during pregnancy, particularly in response to respiratory events and limb movements. Sleep during the first trimester was affected to a lesser extent, with more wake time after sleep onset and less stage 4 sleep when compared to the controls. CONCLUSIONS: Sleep during pregnancy is compromised by higher amounts of wake and cortical arousals leading to sleep fragmentation, with greater amounts of light sleep and less deep sleep. Mood state did not have an effect on sleep. Given the impact of sleep on well-being, this study increases our understanding of the characteristics of sleep during pregnancy, to help recognise when severe sleep disruption may warrant referral to a specialist for appropriate diagnosis and treatment.