She also wore a Victoria Beckham bump-hugging dress to the Christmas Walk this year. If these non-maternity-wear designer dresses are any indication, she probably has a literal team of tailors helping bring her sartorial choices to life. (And we can't even get our dry cleaner to call us back...)
Another possible reason Markle has been opting out of maternity brands: Jessica Mulroney, the duchess's pre-royal-life BFF and royal-life stylist (unofficially, but oft reported on) lives in Canada. This means the two can't meet before every public engagement on the duchess's calendar, and that the pair probably coordinate Markle's wardrobe via Skype, FaceTime or whenever they get together (which could be once a month).
Since the two BFFs can't meet face-to-face very often, it's probably tough for Markle (and Mulroney) to predict which outfit choices will look best for the royal's growing baby bump. At only three months to go, though, it might be time for the duchess to let her palace tailors rest their little thimble-covered thumbs and spring for dedicated maternity wear.
Or just keep on with your glowing self, Your Highness. We're sure the duchess (and her tailor #squad) knows best.