Oo mama, nakakaguluhan talaga yan at totally valid ang tanong mo! So technically wala pa talaga si baby on January 31 when you did the deed, your body was just preparing to ovulate pa lang, kaya yung actual conception usually happens around 12-14 days AFTER your LMP, which means around mid-February for you. The reason doctors count from LMP is because that's the one date na most moms are sure about, habang exact conception day ay mahirap talaga ma-pinpoint unless you tracked ovulation precisely, kaya safer at mas standard ang ginagamit nilang starting point para sa lahat, that's why your due date is calculated from January 15, even though baby technically came into existence a couple weeks after. Totoo na weird ito pero once you get your first ultrasound, they'll adjust the dates based on baby's size, so walang problema!