I think the rationale is not that a woman can't stand when she's pregnant but that in sitting lowers risk for an accident - e.g. the train suddenly stopping une

I think the rationale is not that a woman can't stand when she's pregnant but that in sitting lowers risk for an accident - e.g. the train suddenly stopping unexpectedly or even people just having more people bunched around you when standing. When sitting, you have a more defined space around you and people tend to stand further away from people who are sitting.