You had better re-write your story. Even in medieval times, a woman couldn't give birth to twins and not know that she's given birth to two babies. If she's given birth to one child, she's certainly going to know it when she gives birth to the second. If nothing else, there's the physical sensation of giving birth that's going to give her a clue that she's given birth to more than one child. She can physically feel each child come out. In medieval times, there was no sort of anesthesia (there was no anesthesia at ALL) that could render her unconsicious while she was giving birth.
Up until maybe 70 years ago, it could have been possible for a woman who had no prenatal care to be unaware that she was carrying twins, up until she went into labor and the babies were born. Today, that would be almost impossible. Her doctor is going to know well in advance (if by nothing more than an ultrasound) when a woman is carrying twins.