Abstract Background Gestational diabetes (GDM) is associated with health risks for both mother and child, and is particularly relevant to migrant women and women of African origin. With today’s extensive global migration, contact with the new society and health system confronts the migrant’s culture of origin with the culture of the host country. The question is whether immigrants’ patterns of beliefs about health, illness, and health-related behaviour change over time, as no previous studies have been found on this topic. The purpose was to explore development over time, during and after pregnancy, of beliefs about health, illness and healthcare in migrant women with GDM born in Africa living in Sweden, and study the influence on self-care and care seeking. Methods Qualitative prospective study. Semi-structured interviews, with 9 women (23–40 years), on three different occasions: during pregnancy (gestational weeks 34–38), and 3 and 14 months after delivery managed at an in-hospital diabetes specialist clinic in Sweden. Results Beliefs were rather stable over time and mainly related to individual and social factors. GDM was perceived as a transient condition as health professionals had informed about it, which made them calm. None, except one, expressed worries about relapse and the health of the baby. Instead women worried about being unable to live an ordinary life and being bound to lifestyle changes, particularly diet, developing diabetes and needing insulin injections. Over time knowledge of appropriate diet improved, although no advice was experienced given by the clinic after delivery. The healthcare model was perceived as well functioning with easy access but regular follow-ups were requested as many (decreasing over time) were unsure whether they still had GDM and lacked information about GDM and diet. During pregnancy information was also requested about the healthcare system before/after delivery. Conclusions Beliefs changed to a limited extent prospectively, indicated low risk awareness, limited knowledge of GDM, irrelevant worries about future health, and being unable to live a normal life, associated with problematic lifestyle changes. Beliefs about the seriousness of GDM in health professionals influenced patients’ beliefs and health-related behaviour. The healthcare organisation urgently needs to be improved to deliver appropriate and timely information through competent staff.