When should abortion be allowed and stopped?

As you can probably tell from the title I want to hear from moderates not extreme pro-lifers or extreme pro-choicers. If all your going to say is never abortion is murder or always it is a woman's right to choice please do not respond. I for one think it should be allowed but if a woman has a certain number of abortions (say 3 or 4) over a specified period of time she should not be allowed to have another abortion because she is using it as birth-control for a designated amout of time or just never again. Your thoughts on my idea I welcome all criticism so long as you don't go to far life or choice on me.