If i am 21 weeks when did i conceive?

You get the most from this site when you provide enough data for reasoned answer. Nothing in your post identifies the date posted. You have not provided the date of last menses, sexual exposure or other information that would assist finding an answer. You don't say if this gestation is from an ultrasound or last menses. A simple response is you conceived about 19 weeks ago if this is from menses.