Hi momsh! From what I know, the depo shot is supposed to be effective right away if you get it on the first day of your period, but if it's delayed or you're not sure about the timing, there's always that small window of risk, honestly, a few of us tried relying on timing alone and it stressed us out so much na mas okay lang if you ask your OB to be sure, kasi every body is different talaga. Better safe than sorry, diba? Good luck, and don't worry, your doctor will have the best answer for your situation!