Scientific reasoning competencies (SRC) are one part of science teachers’ professional competencies. This study examines the contribution of three factors to the development of pre-service science teachers’ SRC: the amount of science education classes, the amount of science classes and the pre-service science teachers’ age. The factors amount of science education classes and amount of science classes have been operationalised in terms of ECTS credit points. N = 438 pre-service science teachers from six universities in Germany, Chile and Canada voluntarily and anonymously responded to an established multiple-choice instrument for assessing SRC, which has been developed by the authors and is available in German, Spanish and English. Multiple linear regression analyses show that the included factors explain a proportion of about 9% of the pre-service science teachers’ SRC. The factor amount of science classes is the only significant predictor and can be seen as an indicator of learning science content knowledge. These findings support the assumption of science content knowledge being a prerequisite for developing pre-service science teachers’ SRC.