Abstract Background The unregistered population remains under-researched because of its “invisible” status in statistics. Studies on perinatal health outcomes of unregistered women remains particularly limited. Our objectives were 1) to describe the sociodemographic profiles of women who are not legally residing in Belgium and 2) to analyze the associations of registration status with pregnancy outcomes according to socioeconomic status and nationality. Methods We analysed data from birth and death certificates taken from the Belgian civil registration system, linked with the National Population Registry (NPR). The data relates to all singleton babies born between 2010 and 2016 (n = 871,283), independent of their mother’s NPR registration status. We used logistic regression to estimate the odds ratios for the associations between perinatal outcomes (perinatal mortality, prematurity and low birth weight) and maternal NPR registration status according to socioeconomic status and maternal nationality. Results Over the study period, 1.9% of births were to mothers without NPR-registration. Unregistered women from newer EU member states and non-European countries were particularly disadvantaged from a socioeconomic point of view. Apart from women with a South American nationality, all other groups of unregistered women had higher rates of prematurity, low birth weight, and perinatal mortality, compared to registered mothers (p < 0.0001). Unregistered women from Belgium and EU15 nationalities had particularly higher rates of prematurity, low birth weight, and perinatal mortality, compared to registered mothers, even after adjustment for socioeconomic status (p < 0.0001). The excess of perinatal mortality for non-European unregistered mothers could partly be explained by their precarious socioeconomic situation. Conclusions This is the first study to include data on mothers who were not legally residing in Belgium. Unregistered women giving birth in Belgium are likely a heterogeneous socioeconomic group. Overall, ...