No, faceless fantasy writer, people didn't normally consummate their marriages with 12 y/o little girls in medieval times. Not only was it rather rare for people to marry that young with the exception of the noble class, due to them needing it for politics, but they also weren't stupid enough to try to force a child to give birth. Morals aside, people back then knew how their bodies worked. A heir was nice but they didn't wanna sacrifise their brand new bride and political alliance for one right off the bat; most often the "wife" just lived with her husband in the new country and was raised there for a little while until she was old enough to actually do it. Or she even stayed with her family a little longer if the wedding was held by proxy. Medieval times were cruel, no doubt, but let's not make it worse than it was for the sake of dramatics.
