What can happen if your 16 week old American rabbit gets pregnant?

i have heard that you should wait until they're about 6 months, but im sure if you just left 2 of them together they will mate and you probably wont even know [unless you are monitoring them all the time] that's what happened to my rabbits, they had two litters of babies and we never knew one was pregnant let alone one was a boy. anyways hope that helps!!