Why do they go by your last menstarl period to determine how far along you are even if you concieved earlier?

Question

if i concieved the last week of may, had a period june 9th how could i only be three weeks?

Answer 1

Because They cannot determine the exact day you have conceived-generally, your ovulation is about 14 days after your last period, so you have conceived around that 14th day since day 1 of your last period. If you had a period on June 9th, then you probably have conceived between June 22 and 24th, depending the lenght of you cycle(from 1day of this month's cycle to 1 day of next month's cycle. It is hard to determine exactly when the conception took place...

Answer 2

you can't conceive end of may and have a period in june. you would have to have conceived in mid june.