Participation of pregnant women in clinical trials entails challenges mainly related to concerns about the risks for fetuses. We undertook a qualitative study from June to October 2020 to assess the acceptability of participating in COVID-19 clinical trials among pregnant women in Spain. Phenomenology and grounded theory were used as methodological approaches. Semi-structured interviews were conducted with 24 pregnant women and six healthcare providers. Women were unsure if pregnancy was a risk factor to acquire the infection or to develop severe disease and expressed the limited information they had received, which led to uncertainties and emotional suffering. They had concerns regarding participation in clinical trials on COVID-19, regardless of the drug under study. Healthcare providers alluded to the importance of involving pregnant women’s relatives at the recruitment visit of the clinical trial. These findings may be useful to facilitate pregnant women’s participation in clinical trials.