Abstract Background When a member of a family has been diagnosed with hereditary angioedema (HAE) before a child is born, the question of early diagnosis arises. Indeed, the first attacks may occur at birth. Early diagnosis is complicated by biological issues. Due to the immaturity of the complement system, C1 Inhibitor (C1 INH) and C4 levels can be low at birth, generally in the range of 60 to 100% of adult reference values. Like most complement proteins, their levels generally normalize after one year of life. However, this is not always the case, and we report two counter-examples here. Case presentation A woman with well-documented HAE due to type II C1 INH deficiency gave birth to two children 4 years apart. Functional C1 INH assays performed at 8 and 7 months of age returned normal C1 INH inhibitory activity. However, a genetic exploration revealed the presence of the mother’s pathogenic gene variant in both children. Subsequent monitoring of C1 INH activity at 3 and 4 years of age confirmed a pathological reduction in C1 INH inhibitory activity. Conclusion For the early detection of HAE in children, these cases lead us to recommend genetic testing for the index parent’s pathological variant rather than reliance on results of C1 INH assays.