Hello, Depot shot is very much effective in preventing unwanted pregnancy and to be taken once in 3 months. It has failure rate less than 0.3 per HWY. But there has several side effects like irregular period, prolonged infertility after discontinuation, wt change, headache etc. Pregnancy depends on proper timing of ovulation and unprotected sex. Do a initial home pregnancy test with urine or blood for beta hcg test to confirm/ exclude pregnancy. Avoid stress, take healthy diet, control body wt and need proper sleep. Be well.