So i couldn't of got pregnant the 18th?

Unclear: Your information is unclear, perhaps a follow up to a previous question, but you can only get pregnant during your few fertile days in the middle of your cycle. If you have a normal 28 day cycle that would be around day 14 or so. I don't know what your cycle is so it's hard for me to say if the 18th falls in that area for you or not. But usually around 2 weeks after your last period is the time.