It's extremely unlikely, especially if you're using condoms correctly -- don't tear open the package, use a good quality condom that's unlikely to break, make sure to leave space at the reservoir tip, and also make sure it comes off right after ejaculation and that there's no further intercourse without a fresh one. Condoms are an excellent form of protection from unwanted pregnancy.

As Ian mentioned, if you use a condom correctly, it is very reliable and effective in preventing conception. This means that you should use it from the first moment you have penetration. One mistake some couples make is to have sex for a little while without a condom, then pull out and put it on for the ejaculation. This is very risky because sperm can be released into the vagina through pre-ejaculatory fluid way before ejaculation/orgasm. The other thing to consider is that when a woman is stressed, it can delay her period - so being stressed about not getting your period can make the time even longer.