When using a condom what are the chances of getting pregnant?

Condoms are rated as something like 98% effective, with the last 2% mainly coming from accidents during use. If the condom gets a tiny nick/scratch, it can either split or leak during intercourse. Some men produce a lot of pre-cum which can cause the condom to slip off during intercourse.

And then there's a tiny fraction of a percent of condoms that have gotten through the factory testing and still has something wrong with them.

So, as long as you use them right, and with a bit of care, they do offer very good protection.