Hi, Menstrual cycle starts with menstruation and ends at first day of onset of next menstruation. The menstrual cycle is maintained by the hormones, estrogen and progesterone. There are two phases in menstrual cycle. Follicular phase or proliferative phase, ovulation and luteal phase or secretory phase. Estrogen is the major hormone in follicular phase during which the follicle with egg develops and results in ovulation. Ovulation is the process of release of matured egg which is ready for fertilization. This is followed by luteal phase. Progesterone is the major hormone in Luteal phase. This is the hormone that plays major role in conception. This hormone controls the development of uterus where the fertilized egg gets implanted. Depo provera has contraceptive action via preventing the development of uterine endometrium, prevents the ovulation and also changes the consistency of cervical fluid into thick which inhibits sperm motility. Depo provera injection provides contraceptive effect for up to 12 weeks. It should be taken during the time of menstruation to provide contraceptive effect during that cycle. Alternative method is advised temporarily for women who take injection after seventh day for a period of ten days to avoid conception. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Pradeepraj, General & Family Physician