We found a cat outside and noticed she was pregnant. I didn't want to leave her out there because it is winter her and it gets below zero quite often. Early this morning I found vomit and she has been arching her back whenever she is up. I don't know how far along she is, but I have been able to feel the kittens move for two weeks and it seems like they are farther back. Is there any way to know when labor will start besides taking her temperature?

If possible, the best thing you can do for her is to provide her a place of shelter and warmth-- a garage or even a big box with straw in it  and a blanket to cover the opening will be helpful as well as water and nutritious cat food.  (check frequently because it will freeze and not be accessible to her).  You really cannot tell if she is ready to deliver except you may see her begin to nest in a place. You are more likely to see her nest in a place like what i have described or she may look for a saft, warm, covered area elsewhere.  You would be amazed what cats can do!  She will likely be fine without your help, but it is good of you to be concerned and great if you can offer her distant support!  Good luck. Please let us know how things turned out!