Pregnant Chihuahua....Abort or keep?

I have a 4 year old chihuahua. She was found four years ago by my roomate. We tried to find her owners, but never found them. Shortly after she broke her two front legs. She had surgery and recovered fine. Now, she's pregnant. I have no idea how this happened, i am mortified. The only dog I think that could have gotten to her is my sisters dog. Which is a mut. He's about 25 lbs, Bella is only about 6-7 lbs. I am petrified of her not being able to carry a bigger dogs pups and I worry for her arms. I know in the cold her arms bother her (doc says she has arthritis) but she's packin on the weight, and packing it fast. Is an abortion the best thing for her? Just asking in the mean time before I take her to the vet this week. I'm sittin on nails, thanks to anyone who answers.