Answer
Condoms have been known to break, but it doesn't happen that often. The people who make condoms of today are really careful to make the right sizes so that men don't have the problems of the condom coming off or breaking during sex. Do you know that you can put a condom in a bathtub and fill it, and it will expand to the full length of the bath tub. That's how strong these things are.
If the one getting pregnant is the one wearing the condom, the chances are likely very good.