I don’t know where you grew up, but I consider it a very vulgar term - it is not a “bump,” it’s a baby. Sorry to disagree.
I grew up in Fort Worth, and actually first heard the phrase in my baptist church, much to the delight of the older ladies making a fuss over a mama-to-be. I’m 46 now and in all my years, NEVER come across someone who found the phrase ‘vulgar’, and I’ve lived all over the north Texas area and northern California area—your mileage may vary, though, depending on your region and religion, I guess! [shrugs]
Not sure if you’ve been around a pregnant person before, but when the belly starts stretching (as mine is doing at this very moment) a bump (generally called the baby bump, not just ‘bump’ in most cases, although I knew a woman who called hers “The Bump” before she knew the gender affectionately) indeed does form—a living bump with a beautiful heartbeat (in my case, 2 beautiful heartbeats :*) that is now moving on it’s own when one of the babies kicks/punches inside. :*) One of the most wonderful, glorious feelings I’ve ever felt in my entire life!! :*) Not a single thing ‘vulgar’ about it, IMO.