Asthma is the most common chronic respiratory disease during pregnancy. However, reports of new onset asthma during pregnancy are lacking. We report two cases of new onset asthma during pregnancy following respiratory tract infection, subsequently one case with M. pneumoniae infection and the other case with a combined infection with respiratory syncytial virus and rhinovirus. Case presentation: Both patients presented with the clinical features of an acute asthma exacerbation during pregnancy without a medical history of asthma. During follow up the diagnosis of asthma was supported by spirometry showing significant reversibility and elevated fractional exhaled nitric oxide (FeNO). Patients were hospitalized and received supplemental oxygen, treatment for an acute asthma exacerbation with systemic corticosteroids, high dose inhalation therapy. These therapeutic interventions subsequently led to a good outcome for the mother and newborn in both cases. Conclusions: New onset asthma should be part of the differential diagnosis in pregnant patients with respiratory symptoms, particularly in case of mycoplasma infection. Diagnosing asthma during pregnancy can be challenging. In these circumstances, additional diagnostic tests (like inflammatory biomarkers FeNO and blood eosinophils) can be helpful to support the diagnosis.