oral contraceptives (pills) work by inhibiting ovulation and implantation as well as altering the quality of cervical mucus. In matching questions, 'Prevents ovulation' is often the specific characteristic used to distinguish pills from IUDs (which are often matched with preventing implantation or increasing phagocytosis) and barrier methods (which prevent sperms from reaching the cervix). 'Semen contains no sperms' corresponds to vasectomy.