OK, bearing in mind I'm no medical expert!!! Firstly it indicates you're ovulating. You can only get pg when there's an egg available and that only happens at one time in your cycle - when you ovulate. So you know it's the right time when you start to get ewcm (eggwhite cervical mucus). Secondly it is actually needed for the little swimmers to survive! During the rest of your cycle your cm is actually 'hostile' and the sperm can only survive for a few hours. When you've got ewcm, it acts as a medium to help transport the swimmers to the egg and provides nutrients etc to keep it alive. You normally get ewcm for a couple days in the lead up to ovulation (in your 20's usually about 4 days, less by your 30's allegedly), and in ewcm the sperm can survive a few days until the egg has actually popped out! I'm sorry for the poor explanation but hope it makes sense. If you google this kind of thing there are some better explanations!!!! :lol:
Also fyi, once you've ovulated (normally considered to be on your last day of ewcm) your period will always arrive approx 12-16 days afterwards. So if your cycle is erratic, then ovulation may happen any time, but regardless of how erratic your cycle is, your period always comes 12-16 days after ovulation. So if you think you've ovulated (from your cm or if you're using ovulation sticks which you pee on), and your period hasn't arrived 16 days later it's def time to try a preg test!!!