This is a tough one. I had to have my mommacat aborted after I trapped her. I don't know how far along her pregnancy was, but she was so depleted and malnourished it was really necessary.
I still HATED to do it.
Can you trap your momma and bring her inside to have her kittens? If she is anything like my momma, you won't see the kittens until they are 8 weeks or so old, and then you will have the job of trapping and taming them all!
At least if they are born in your house, they will all be tame and so much easier to adopt out.
Being aborted is not the worst thing, unfortunately. The world is a cruel place for little kittens being born outside.