Retraction Note: Improving usability and pregnancy rates of a fertility monitor by an additional mobile application: results of a retrospective efficacy study of Daysy and DaysyView app. - Koch MC; Lermann J; van de Roemer N; Renner SK; Burghaus S; Hackl J; Dittrich R; Kehl S; Oppelt PG; Hildebrandt T; Hack CC; Pöhls UG; et al. (Reproductive health, 2019)