I am also 19 weeks and I say I am 4 months pregnant.
I change months on the anniversary of my due date based on how much time is left in a 9 month pregnancy. (I have 5 months to go so I am 4 months pregnant).
So I am due 11/6. On July 6th I will start saying I am 5 months (b/c I have 4 months to go) and on August 6th I will say I am 6 months (b/c I have 3 months to go) etc.
It just makes more sense to me...