That’s exactly what happens. There are girls that were virgins at 15 and pregnant at 16.
It doesn’t require a build up to get pregnant. Your first time is enough to get pregnant.
If they were sexually active since they were 12 and only got pregnant at 16 that’d be a miracle. More times than many they get pregnant the moment they start having sex.
Look up research online about the ages that pre-teens start getting sexually active, it's not 15/16, it's earlier.
Where did you grow up? In the West, everybody knows that teens start dating from age 12 and then they get progressively more inappropriate with each other the longer they are together.