Possible talaga, pero mas mababa lang ang chances compared sa ovulation window! Sa 25-day cycle mo, most likely ovulating ka around day 11-13, so day 18 is usually after peak fertility na, but sperm can live for a few days kaya never say never talaga. If you're trying to conceive or avoid, better talk to your OB about your specific cycle, a few of us mums got surprised pregnancies even outside the "typical" window kaya mas reliable mag-track with a professional, especially since cycles can be unpredictable din!