The Depo shot can prevent menstruation because a menstrual period occurs when the hormonal levels drop under a certain value. The Depo shot elevates the hormonal level so a period (menstruation) can be prevented. Women who use the depo shot as birth control either have no periods or experience rare and irregular periods.

You can’t miss the mark if you perform a pregnancy test. Then you'll know for sure if the lack of a menstrual period is caused by the birth control, or by pregnancy.