How do i stand up to my overbearing mother?

Hi
Im 27 and live with my children and my partner. Before i met my partner my mum helped me a lot and still does as in she sees to the children when we are at work and will wait in for deliveries and things.
The thing is I have just found out that im pregnant again and although its planned and we are very happy I have a big dark cloud over me about telling mother! She is constantly making me feel like all my choices are wrong – she even had a arguement with me last week about what year i left school (I think I would know) and i know as soon as i tell her i will have to listen to her going on at me as she will be seeing to the children next week – she is not by the way going to have to see to them once my new baby is born as im not going back to work!
Any advice would help but not the obvious – tell her as im running out the door which i already thought of!
Thanks