I think it's a matter of how you personally define the word "mother".
Once I gave my girls up for adoption, I was no longer a mother by my own definition. I consider myself nothing more than a birth mother even though I raised them for a portion of their short lives, and until this child is born and I am raising her I don't consider myself a mother by "standard" definitions, merely an incubator.
But like I said, I think it's all a matter of personal definition. There are plenty of peopke who say I'm still a mother simply because I did raise Em and Annie for a period of time.