Well, not exactly, - 6 months is the earliest considered a good time to start breeding intentionally, but the youngest of my rabbits that accidentially got pregnant was 14 weeks, they are about 10lbs grown up - smaller breeds can mature earlier. It's not a sure thing at that age, but better act on the assumption that she's pregnant. No way to tell for sure other than to wait somewhat over a month (and prepare a plan for what to do with all the rabbits)
Anyway, it doesn't matter much if they get neutered anyway, just have it done really soon. Tell the vet about it.