There really aren’t any signs of pregnancy until they have babies. They will secretly make a nest and give birth and you just won’t know ahead of time; you’ll just find a nest one day and a very tired mom…unless there are complications and you might have mom in the e=-vet in the middle of the night.
Nipples don’t drop until they are born and their tummy doesn’t get big at all really. The only signs you might see is she acts aggressive or territorial, which at the teenage time, you would see anyways.
Any rabbit who is not spayed or neutered needs to be kept separate until they are fixed. Constantly people have two girls or two boys who through immaculate conception (Re: missexing) find babies one day. It’s cute for a couple of hours. Finding homes and providing care (vet care$) is expensive and time consuming, not to mention the abundance of rabbits in shelters who already need homes. So if you have more than one unfixed rabbit, separate now until they are fixed, and if you suspect one is pregnant, see a vet as soon as possible.