If a woman's periods are regular and she experiences no bleeding in pregnancy. Many women's periods are not regularly, especially so if many woman are malnourished. Moreover, bleeding in pregnancy (esp early pregnancy) is fairly common. Thus, if you read older gynecological manuals, they clearly claim that pregnancy lasts some range of months. Most people realized that it was centered around 10 months, but there were many doctors claiming that the ranges were much higher than we'd accept today, because they had no reliable method to test for conception.
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.
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.