Menstruation is a result of ovulation. It happens when the egg released by the dominant ovarian follicle is not fertilized. As the egg does not implant itself in the uterus, the endometrium deteriorates and is shed.
When you’re on hormonal birth control, it doesn't work that way. These contraceptives (the pill, the vaginal ring, the implant, the patch and the intrauterine device) are made from female hormones like estrogen and progesterone. While some combine both hormones, others do not. In order to prevent a pregnancy, hormonal contraceptives inhibit ovulation and thicken the cervical mucus. Without ovulation, fertilization is not possible and, if the mucus is thick, the sperm will have a much harder time penetrating the egg. In short, hormonal contraceptives interrupt the normal menstrual cycle.
It is thus incorrect to say that those who use hormonal contraceptives experience menstruation. As we have already explained, menstruation only comes from ovulation. The blood that comes out of the vagina under contraception is a false menstruation induced by the contraceptive hormones in question. It is called withdrawal bleeding, which involves the organism resetting and the endometrium falling apart when the administration of hormones is interrupted. While this bleeding has no particular benefits for the body, it works on the psychosomatic plane, reassuring women they are not pregnant.The blood from withdrawal bleeding may be slightly different from normal menstruation and is usually less heavy.