Females get pregnant and give birth at 12 in many species, yes. Human women, not so much. Starting menstruating that young is a recent change; nutrition was poorer back then, and menarche usually didn't set in until around age 15-18.
Even now, with menarche around 11-12, the girls' bodies are definitely not ready to give birth that young, and many (in cultures that allow marriage that young) die during pregnancy or childbirth. Some villages in areas like that consist of young girls and older women, as most women of childbearing age have died during their first, second, third, fourth, fifth, sixth, seventh, eight, ninth (etc) childbirth. You know why it was common for older men to marry young girls? BECAUSE THEIR PREVIOUS WIVES HAD DIED IN CHILDBIRTH!!!
What the FUCK do you know about complications back then? Even today, with advanced medical science, many women suffer long-term complications after pregnancy and childbirth. Incontinence, either urinal, fecal or both, are fairly common complications. Particularly girls giving birth too early suffer from something called genital fistulas. Google "Fistula Hospital", Jonny-boy, if you want some real-life horror stories. A common complication back then was to be so damaged during childbirth, that you were not able to have any more kids, which might make your husband abandon you, and you to either starve to death or go into prostitution.
There have been girls as young as 2-4 that have gotten pregnant. Getting pregnant is not the hard part, asshole! Getting the kid out through the tiny pelvis IS. Do you have any idea what a childbirth is? It's sort of like passing a tennis ball through your urethra. Google "Symphysis pubis dysfunction", while you're at it, and imagine how it would feel if your pelvic bottom would start to disintegrate...