Earlier quoted context omitted.
Up until very recently, there was very little knowledge that pregnancy lasted nine months... even among the 'learned'. Old pregnancy manuals talk about pregnancies taking anywhere from a few months to many many months. Without ultrasound and HcG tests, there is very little indication a woman is pregant until she starts showing or feeling the baby (and even then, a new mother may not even notice until she's showing an…
Quick comment about your first paragraph. There is a really strong signal that a woman is pregnant in absence of period.
If you bleed like some women do during pregnancy, then you may have no indication you're pregnant until you obviously show. It's unlikely, but not impossible, and the doctors had no objective measure by which to say otherwise unless they could feel or hear the baby.