Hi K.,
I have delivered several litters of puppies - when my mom was alive, we bred our Sheltie's - not a Chihuahua, I know, but generally the same.
I did find a web site for your specific need. Looks kinda cheesey, but I read thru the info and the woman is 'right on' with her facts. http://groups.msn.com/ChihuahuaNook/cn1.msnw
Though I think the website may be foreign-origin... because she mentions a couple times that she's not sure what "X" product is called in the US.
you don't say if you bred her intentionally or if it was an accident... but it's important to know if the dad was also a chihuahua...it can greatly affect the size of the puppies if not. Also have you notified your vet of your pending arrival? You'll want to make sure you have a resource in the middle of the night should a problem arise. So have an emergency plan in place. I've learned that the smaller the dog, the more difficult it is for them to whelp puppies without assistance. You will also want feedback from your vet as to what constitues an emergency. Has your vet palpated your dog to see if she can guess how many puppies she's expecting? ultrasound probably isn't necessary but a trained person can typically manipulate the abdomen and guess how many are coming.
What do you need?? I would have a lot of towels on hand (paper and otherwise),... lots of newspapers, a quiet place for her to deliver in a box with many newspaper layers so you can remove a few layers after each puppy, because it can be messy and sometimes bloody. Hemostat if you have one, to pinch the cord if she does a sloppy job of biting it off from the placenta - otherwise you can just pinch with your fingers and cut it (about 1/2 inch from the abdomen - no closer or you'll invite hernias). plenty of fresh water for mom to drink - laboring is hard work... you know first hand I'm sure how annoying it is that nurses won't let you drink fluids while you're panting and blowing - like an ice chip is really effective enough :) LOL
If you have a scale that measures in ounces,... great! not entirely necessary, but nice to have so that you know their birth weight, and weighing them in a few days will ensure they are getting enough milk and gaining weight. You'll want a pad of paper to make notes of her delivery. When puppies were born, their weight, male or female, and how long between puppies.
Not sure how squeamish you are... but if you plan to take a very active role, expect a mess. Your dog may vomit, tremble, poop... placenta bags will burst open, and amniotic fluid will spill out, and also it IS NECESSARY that she eat at least one placenta but no more than two or she will have serious diarrhea. The act of doing this i'm told is to begin milk production. If she won't eat at least one, you'll need to call your vet. I found typically that we had to stop them from eating them.
I am sure I'm leaving stuff out... you can feel free to private message me if you have any questions. I've delivered well over a dozen litters - no expert, but experienced all the same.
You can tell when your dog is labor typically... her behavior will change, and her temp will drop. their temp is normally around 102, and it will drop to 99. She may dig and scratch, she may not eat, she may seem frantic as it gets closer to delivering... try to keep her in the 'whelping box' and encourage her to lay down and rest while she labors, be careful about allowing her to go out to the bathroom once labor begins... sometimes they have to go and if she's clearly not in distress let her go. If labor progresses for a long time she may want to eat, and that's ok too.
Expect her to shake and tremble and not know what to do with herself :) And FYI - our dogs always seemed to go into labor in the evening... well into the morning hours... so expect you'll be up with her if you plan to assist.
And then sometimes even the best laid plans go out the window and you wake up with a litter of puppies at the foot of your bed and they're all nursing and all seems well. :)
I would suggest putting her on puppy food starting now, until she stops nursing. extra calories for nursing.
Oh, and she's likely to lose some hair... this is normal too. Our shelties always looked mangy...
Good luck! sorry for the length :) hope i've helped a bit.
EDIT:
I do want to add as an afterthought... spaying your dog is the healthiest choice for her long term health anyway... I dont want to shove it down your throat though... it's ultimately your choice, but your vet can advise you on the health benefits of spaying, all dog shelters/euthanizing aside.