Just adding this article here from the ACOG about how tenuous the definition of viability is and how few babies actually make it when delivered in the 20-25 week mark. We love to focus on those miracle babies who do make it this early but it is definitely not the norm.
He most certainly did aquire his knowledge through dissection. The real genius here is that he was able to see these structures in death and then “see” and reproduce it in his art of the living. Bodies in death do not look the same as bodies that are living.