An 11 month pregnancy is common enough...if the mother is a little confused about the date of conception.
My mother was told, growing up, that a 7 month baby was more likely to survive than a 8 month baby. Now, we know that's not true, but she insisted there were many cases of that happening at the time. I asked, 'How many of those 7 month babies were born 7 months after their parents' wedding?' and her mouth snapped shut.
Of COURSE they were comparing full-term babies born less than 9 months after the wedding, and true 8-month babies!
My sister read a historical account of a girl who was delivered of a big, heavy baby less than 7 months after her wedding. Her father was outraged...no way that could be a premature child! The doctor, taking pity on the young couple, told her father that indeed it was true that a big premie like that was a massive risk to the mother's health, and he'd better get to praying that his daughter survived the delivery. The father repented, and quit reviling his daughter and SIL for having conceived the child before the wedding. The doctor and his nurse carried out the charade so that the new family wouldn't be victims of gossip. So perhaps there were other doctors who distributed misinformation to angry in-laws, and that's how the 7-months myth came up in my mother's home town.