Please explain, why do they base how far along u are off of ur last period?

My last period was The 22nd so.in pregnant terms I would be a month... but technically I concieved on june 5th so I would only be 2 and a hafe weeks... why dont ppl go.off of conception date?|||Because many people have sex more than once a month, therefore they may not know which time they conceived. It is a given it would have been after the last normal period, so they base it on that day, and count forward 9 months, plus one week. So "in pregnant terms" you are 3 weeks and 2 days along. Not that far off. Your due date is only a guess. It is understood you could go a couple of weeks early or a couple of weeks later. Babies aren't Butterball turkeys, either. There is no pop-up timer. They'll come out when they are ready. So just expect it sometime around your due date, not exactly 9 months after you conceived, or 9 months plus one week after your last normal period. Have your hospital bag packed a couple of weeks before hand, cause it can happen anytime then. Congratulations!