There is generally a 2% to 5% chance that you can get pregnant while on hormone therapy, assuming normal conditions. HOWEVER, there are a variety of ways that you can increase the chance of becoming pregnant, including the influence of other medications and, presumably, diet/lifestyle. If it’s very important that you don’t get pregnant, it’s probably best to use condoms and hormone therapy. But if you’re having sex with someone that you are in a relationship with, you might be willing to use just hormone therapy.
I know a couple that is having a baby in a month who became pregnant while on birth control. It can happen to anyone.