I had a friend from Shanghai who did this. It’s completely legal, you can have a proper tourist visa and be pregnant when you enter the USA, there are hospitals in SoCal that even cater to anchor babies and will express a passport for them so the parents can return with the baby shortly after birth.

That was back in the early 2010s, I don’t think it was prevalent then (I just had too many friends with the money to do that). I don’t think it is common now because Chinese citizens have more confidence about China and so aren’t looking for backup plans anymore.