How Accurate is Eve's Period Prediction?

According to eve my period is late, but I'm not exactly sure when my period comes- I've never really tracked it. I normally get it the third week of the month, but eve says I should have it every 4 weeks. I'm just not sure. I'm worried about it because I recently had sex (he pulled out) and I'm fairly young, so I really don't want to be pregnant... So how accurate has it been for you??