I don't think your weight has anything to do with being able to find out the baby's sex. It's all about the baby's organs being developed or not. By 19w3d, chances are you WILL be able to find out. I mean, there is a slim possibility you wouldn't be able to, but like I said, it would have to do with the baby's development, not your weight.
Also, that woman needs a punch to the throat.