What are the odds of pregnancy while on period?

I had sex on my period. I take a BC pill every night, but I started my new pack a day early and had sex the next day. There was only pre-***, not a full-blown ejaculation. I realize there is always a chance for pregnancy, but I was wondering if anyone could give me a estimate or a likely percentage for my odds of pregnancy.

There is always a chance of pregnancy everytime you have sex, regardless of whether you were on your period or not. You have the same likelihood as someone who wasn't on their period.