Well, it depends on how pregnant the cat is. As much as we hate it, at the shelter, if the mother is not far along, we will abort the kittens by spaying her. It seems less cruel than to wait until they are born, only to be euthanized when there are not enough homes.
I think that's a decision you will have to make yourself. Are you sure she's a stray? If she had kittens, could you find forever homes for them? If so, wait until the kittens are weaned, but you'd have to adopt her or foster her, at least.
Zalensia's right. In rare cases, a female will be fertilized when the kittens are only a few days old. I hope you can help her or get her to a no-kill shelter. We have a list on this page. If you know of more; if those in your area are not listed, please add them. That way, we can keep it up to date.
It's so sad to think of a loving, trusting cat being abandoned or lost? It's hard to think of, isn't it?