Oh I just noticed you're 17 so you're most likely trying to AVOID having a baby. In that case the best thing to do is to refrain from sex because even using protection isn't 100% effective in preventing pregnancy or preventing you from contracting STDs.
You cannot get pregnant anytime of the month. The only time you can become pregnant is during ovulation which lasts up to 48 hours. So you only have about a 2-day window in any given cycle where you can conceive. But as AnnieBrooks explained VERY WELL (thank you for that!!!) sperm can live in the vagina for almost a week. So you can most definitely become pregnant even if you do not have sex at your peak fertile period. Some women have gotten pregnant from having sex during their period.
MissingLink also gave good information! You don't necessarily have to insert your fingers. When you use the restroom you will find mucous on the toilet paper. The clear, stretchy mucous mean you are most fertile and most likely ovulating or about to ovulate. Mucous keeps the sperm alive as the vagina is very acidic and sperm is so very delicate. So if you are trying to conceive MY personal rule is to have sex anytime you have cervical mucous.
if you don't know when you ovulate and want a simple way of knowing when you are fertile, simply check you mucus (use two fingers, give yourself a bit of a wipe inside and then watch as you move your fingers apart).
If it is stretchy (some people compare it to raw egg white) then it is sperm friendly and you are at the most fertile part of your cycle.
Anniebrooke explained it very well. A womans fertile period depends on her particular cycle. But generally speaking the fertile period would be several days around mid cycle.
Yes and no. Sperm can live up to four (some say five) days in the woman's reproductive tract. That means if the couple has sex on the 5th day of the woman's cycle, she could ovulate as late as day 9 or even day 10 and get pregnant from the sperm still hanging around waiting from the sex on day 5. Day 9 and 10 are relatively common ovulation days for many women.
The "average" woman's cycle is -- day 1, period starts. Day 14, ovulation happens (the egg lives for about 24 hours). Day 29, new period begins (a new day 1.) However, a lot of women ovulate early. If a woman has a short cycle (say her period usually comes on day 26) she might ovulate earlier. (Or she just might anyway. My little sister always ovulates on day 8.)
So, in answer to your question, a woman is fertile the day she finishes her period, and that whole following week or 8 days (after her period ends) is probably her most fertile window (i.e., from day 6 to day 12 or 13). The sperm can wait around for her to be fertile, for up to five days. Her chances are highest when she has sex at the time she is ovulating.
You can either use the temperature method or ovulation test kits to confirm you are ovulating. That's the best way to know your own personal fertile days in relation to your cycle.
Good luck!