Ay, congrats sa first time mo with family planning! Based sa standard guidelines, the injectable ay effective agad if given during your period or within the first 5 days of your cycle, pero kung hindi ka sigurado kung kailan exactly ang timing mo, better check with your OB just to be safe kasi bawat body ay different. A few of us moms sa group namin were confused din about this, so walang malisya sa pag-clarify with your doctor, para totally peace of mind ka lang!