Congenital fibrinogen disorders (CFD) include several types and subtypes of fibrinogen deficiency, resulting from monoallelic or biallelic mutations in one of the three fibrinogen genes. While it is relatively easy to make an accurate diagnosis based on activity and antigen levels of fibrinogen and genotype, prediction of the clinical phenotype is challenging. Even among patients with the same genotype, the clinical features are heterogeneous and unpredictable. The development of next-generation sequencing rises the possibility to integrate genetic modifiers to explain the subtle relationship between genotype and clinical phenotype. A recent development in integrative hemostasis assays can also help in the determination of patients at risk of bleeding or thrombosis. In this short review, we go through these topics and explain why CFD could be considered an oligogenic rather than a monogenic disease.