In this paper, we investigate the effect of TDD, as compared to a non-TDD approach, as well as its retainment (or retention) over a time span of (about) six months. To pursue these objectives, we conducted a (quantitative) longitudinal cohort study with 30 novice developers (i.e., third-year undergraduate students in Computer Science). We observed that TDD affects neither the external quality of software products nor developers' productivity. However, we observed that the participants applying TDD produced significantly more tests, with a higher fault-detection capability than those using a non-TDD approach. As for the retainment of TDD, we found that TDD is retained by novice developers for at least six months.