Can this happen....?

I say that's crap too. they cant just take your baby your husband is right they would look into the situation first . If shes having trouble gaining weight like that you should take her to a pediatrician so they can look further into whats going on there. since you said the supplementing with formula is working give that some time. don't worry about it if they were gonna turn you in they probably would have already. if you're still worried about it go somewhere else to get her weighed (or buy a baby scale for your house they should be cheap on ebay) Good luck and i hope your baby is well.