How is the person at the bar supposed to know, particularly in the earlier stages? It's not something you can police without being creepy.
Anyway, as much as I'd like people who are planning on carrying to term to use some common sense when it comes to drinking while pregnant, there's no point in 'white knight-ing' on their behalf. And most bar staff just want to get on with their job.
How is it white-knighting?
Drinking alcohol doesn't really hurt the women directly, it hurts the fetus/baby.
It's not. Most bar patrons would be very uncomfortable seeing a visibly or known-to-be pregnant woman drinking in their midst, and rightly so. In the interest of providing customers with a welcoming environment, bartenders should find a way to ensure that pregnant women are not drinking.