The possibility for pregnancy will depend upon the number of days your menstrual cycle lasts. If your cycle lasts 22 days, then the 28th of February is the more likely day for conception. But if the cycle lasts 25 days, then the 3rd of March is more likely. You can’t be 100% sure if someone ejaculated inside you or not. Emergency contraception (“plan B”) is not 100% efficient, either. The most efficient means for determining paternity is to request a DNA sample from both men to compare wit the baby.