I think you're not getting any answers because no such rules exist. I've travelled while pregnant-but-not-showing. I flew into the States at about 3 months, for example. I fly into the States or Canada (mostly from each other, but also from Europe, Africa, and Australia) many times a year. At no time over the past 20+ years has any paperwork or people asked me if I was pregnant or not, and I've never heard of anyone being asked for paperwork or proof of coverage, other than the airline wanting a doctor's letter for flying past 7 or 8 months. That's just mostly so they can be sure they won't have to deal with labour during the flight.
The Canadian government information for visitors site does some arm waving about how you shouldn't have a serious medical condition that would be a burden on the country, but doesn't mention pregnancy at all. It's mostly focused on not being a criminal and throws health reasons in towards the end. Everyone has to have a plan to leave and enough money to cover them during the stay.
Enjoy your visit!