There's a slim window, and there will always be a chance that one trooper makes it through at the right time. Most women are at their most fertile 11-15 days after their period begins. If you keep track of your cycle then you should be able to work out your windows for safe sex. Then there's always a condom which has spermicide on it in case any little buggers slip away

Have you considered birth control?

Because you never really know when you're going to ovulate unless you've been keeping track of it for month, there's really no set window for sex.

If one form of birth control isn't enough, then it's always good to double up (Like birth control pills condoms).

There's always a chance, so you need to take steps to make that chance as small as possible.

Birth Control Pills Period.

It worked really well with my ex before. Never had problems. I didn't even have to use a condom..lol