Background: Healthy eating behavior throughout pregnancy and postpartum is important. This study aimed to investigate the perceived sex-specific importance of determinants of changes in eating behavior during pregnancy and postpartum. Methods: Fifty-four determinants were rated by first-time parents (n = 179) on their impact. Experts (n = 31) rated the determinants in terms of their modifiability, relationship strength, and population-level effect from which a “priority for research”-score was calculated. Results: During pregnancy, the three highest rated determinants by women were “health concerns”, “physiological changes”, and “fatigue”. Men perceived “health concerns”, “health consciousness”, and “influence of the pregnant partner” as important. Postpartum, the three highest rated determinants by women were “adaptation to rhythm of baby”, “baby becomes priority”, and “practical constraints because of the baby”. Men perceived “adaptation to rhythm of baby”, “fatigue”. and “(lack of) anticipation” as important. According to the experts, “professional influence”, “food knowledge”, and “home food availability” received high priority scores for both sexes and during both periods. Conclusions: Priority for research and interventions should go towards tailored family-based approaches focusing on food education in a broad sense taking into account aspects such as health consciousness, self-efficacy skills, and the social and home food environment while being supported by healthcare professionals.