Well, first of all, as many others have said, she really should be at least 5 pounds before you consider breeding her, but you should know that there are 5 lbers, heck, there are TEN pounders that have trouble whelping litters. Anything can happen and you always have to be prepared that you're taking a risk to lose your girl if you breed. The best thing to do is to research her lines and see if her parents had any trouble whelping litters, and if there are any serious health problems in her genetics (even if they don't show up in her they could be further back) such as liver shunt. Then, as Woogieman mentioned, her pelvic bones must be wide enough for her to deliver a pup. My mentor taught me to stick my thumb in the groove of the pelvic bone. If it fits in easily and you have a little wiggle room, she should be able to have normal sized puppies ok. If it contours to your thumb or you can't get your thumb in there at all, you probably shouldn't do it.
But, if you DO decide to breed her, the best thing you can do is to RESEARCH RESEARCH RESEARCH, and find a mentor to help you. The best way to learn is hands on experience, so if I can find someone who trusts you to sit in on a litter being born, that's the best way to do it. There is also LOTS of good information on this site, you just have to look around. Good luck!
