Belgium has recently modified the way ‘sex’ is legally certified for trans* people following the principle of self-determination. However, like in most countries, this modification has not changed the way ‘sex’ is determined for all members of society, being limited to trans* people. How is sex legally certified for different categories of people in Belgium and what are the effects thereof? Drawing on the theoretical perspective of discursive psychology (DP), we consider sex/gender categories as something constructed in discourse – in this case, legislative discourse – and not as an essentialist and pre-discursive reality. Categories are thus an effect of discourse: what discourse accomplishes or constructs. Based on this premise, this paper aims at elucidating the ideological effects and practical implications of the legal certification of sex in Belgium. The effects of the legislative discourse are ideological in the sense that they establish social norms, in this case, regarding sex/gender, leading to practical implications in everyday life. To elucidate these effects, we first identified Belgian civil law on the legal certification of sex since the establishment of Belgium as a sovereign state. We then applied a DP-inspired analytical device comprising a qualitative content analysis and the examination of content variability. The variability of discourse is a key analytical tool to elucidate the effects of discourse since these are not directly observable. The results show that the analysed legislation constructs women and men as natural categories. This is carried out through: 1) the establishment of a distinction between the population at large (unmarked) and those ‘outside the norm’ (marked subjects: ‘transgender’, ‘intersex’); 2) the lack of legal regulation of the attribution of sex markers at birth, being taken for granted; 3) the regulation of the latter only in ‘abnormal cases’ (called ‘children suffering from sexual ambiguity’); and 4) the fact that gender identity is recognised as a criterion for ...