Not in the vast majority of cases, but I'm not the one who brought up pregnancy in the first place. I think JBs are best to avoid getting pregnant beyond any doubt, it's just not that dangerous if they do, at worst it's a bit more dangerous than if they were a bit older (at best it's less dangerous than it would be if they were any older), but still much less dangerous than if they were in their thirties. There is no research on early teens that I'm aware of though, only mid teens (15+).
As I said though, we have birth control that is very effective, JBs with proper sex education and access to contraceptives are not going to get pregnant if they don't want to, in the vast majority of cases.