Background: Sleep has a complex nature that is thought to make it a risk factor for many health concerns, which have recently included poor pregnancy outcomes. Aim: Studying the association between sleep and poor pregnancy outcomes in pregnant women. Methods: To achieve this aim, several studies were done. First, the literature was searched to examine and critically evaluate the quality of current evidence in regards to sleep and pregnancy outcomes. Second, the latent complex nature of sleep was defined using latent class analysis and the UKHLS data set before examining the association between the generated patterns and socio-demographic features and health. Third, sleep events present in the UKHLS sleep module and the generated latent sleep patterns were examined in women from the UK population who were presented in the UKHLS study, and in women at risk of gestational diabetes (GDM) presented in the Scott/Ciantar study, in relation to poor pregnancy outcomes. Results: In the literature there was ‘positive’ evidence of an association between sleep and poor pregnancy outcomes. However, the evidence suffered from limitations, and the complex nature of sleep was not considered. Our definition of sleep as a latent variable revealed six latent sleep patterns which were associated with individual socio-demographic features and health. Sleep events and latent patterns did not always elevate the risk of poor pregnancy outcomes in women from the UK population or women at risk of GDM, as sleep lowered the risk on some occasions. Conclusion: Sleep might increase the risk of poor pregnancy outcomes, according to evidence from the literature review and the two empirical studies. However, the current evidence had many limitations, and further research is required in this area.