Are you serious? How about my credibility as a medical professional? How about the fact that woman slipping and falling 2 feet (distance from hips to ground) can cause fetal demise? That's not even taking into consideration the amount of jostling she would experience on an 80 mile journey. This would take 2 weeks of horseback travel. Even if fetal demise did not occur, it is almost certain horseback riding would certainly induce labor*. *Riding horses during pregnancy
If you really want/need references stating that horseback riding is not recommended while pregnant: Can I go horse riding while I’m pregnant? Horse Riding During Pregnancy
Of course, a simple google search would have yielded similar results.
Sounds like you are making an appeal to authority. It's also your responsibility to justify your claims, not mine to justify them for you.
What you supply is good evidence though. It's not completely convincing as we know nothing about how Mary made the journey, but I agree it is a serious factor. One thing to consider is that the audience for this would have been well aware about travel conditions - would they have bought the story of a very pregnant woman taking this kind of journey?
More important I think is the point about censuses - I agree this sounds like a device by which Joseph and Mary could be placed in Bethlehem. So I'm fundamentally with you.