Desogestrel mini pills prevent ovulation as well as combined pills do (over 97% effective at preventing ovulation.) Norethindrone mini pills, which are older and less commonly prescribed now, sometimes allow ovulation but they have many other mechanisms that prevent pregnancy such as thickening the cervical mucus to prevent sperm from reaching an egg and thining the uterine lining so that an egg can't implant. Used correctly its just as effective as the combined pill