You can get pregnant at any time. The odds are higher during the middle of your cycle (two weeks after the day your period starts) but you can get pregnant from sex that occurs well before or after your peak fertility.
Okay here's the down low! Typical cycle- Day 1 is the first day you start to bleed. The last day of your cycle is the day before you start bleeding(usually day 28).
Day 14 is the day (typical 28 day cycle) you might ovulate. The ova can live for about 24 hours once you ovulate. The sperm can live up to 4 days in your fallopian tubes. So day 10(ish) is the start of being fertile. If you have sex on day 10, generally speaking, you could get pregnant when you ovulate on day 14. Ovulating on day 14 and then having sex on day 15, you could still get pregnant.
Did I confuse you? Hope not. The idea is to figure out when you ovulate. Get a good book. I had the Shettles method referred to me and I love it. I have learned soooo much about myself.