There are some golden posts here that describe the menstrual cycle in detail. Or you can go to a search eninge and look for "menstrual cycle." Or any female health book should cover it.

Menstruation and ovulation are caused by the particular mix of estrogen and progesterone in your system. At one mix, they cause ovulation and at another, menstruation. In between, they are changing mixes. Starting from day one of your period (assuming a 28 day cycle), the likelihood of ovvulation increase up to day 14 and then decreases until your next period. Considering that you are fertile, realistically, for about two day and sperm can live in your reproductive tract for up to four days, that covers a big part of the month that you are at risk.

If your period is perfectly regular, then your ovulation is pretty predictable. If your period varies or is strung out over a week or more, then your ovulation is unpredictable.

Your initial premise of having sex two weeks before your period is almost guaranteed to make you a mother. Frequently having an uncovered penis in your vagina will do the same. Use some dependable form of contraception. For teens, I recommend condoms - every time - with spermicide in the vagina.

It is never safe to have unprotected sex. Women have gotten pregnant during their heaviest flow.

I think they say you ovulate between like anywhere from day 10-14 of your cycle. Day one being the first day you start bleeding.
All women are different and dont always follow this criteria. Thats why its not a safe way to determine "safeness" or not.
Technically you could get pregnant at any time during your cycle, even while on your period. Not as likely but it still can happen.
Maybe Brandye will have a more "doctoresque" reply