You are actually 4 weeks pregnant by the time you miss you period. They count from the first day of your last period, since that is the begining of your cycle, and nobody knows what day you really actually concieved. Even if you know the day that you had sex and got pregnant, you may not have actually concieved for a couple days afterwards since sperm can live inside of you for awhile, and you may or may not have been ovulating at the time of sex.
they say you conceive, exactly 2 wks after the 1st day of your period,
they dont know when you concieve, and that is why they count from the first day of your last period. Everybody can conceive at a different time of their cycle. It all depends on when they ovulate, and when the sperm fertilizes the egg. It is not always exactly 2 weeks after the first day of your period. Everybody is different, but the average ovulation time is day 14 of your cycle, for somebody with a 28 day cycle, but everybody is different. Some women can even ovulate at the end of their cycle, or the begining... Not even in the middle. For all of this confusion, is the reason why they count from the first day of your last period.
You have more chance of knowing the date of your last period (for obvious reasons) rather than knowing when you ovulated.
It makes it easier to count from your last period and give and estimated date of delivery, and this is why your baby could be born (in normal circumstances) 2 weeks either side of your due date.