Your baby is really tiny! I would be scared to death to have her put under too.
I have 2 girls that have yet to be spayed and they are 6 lbs. at 15 months and 5.5 lbs. at 10 months. Both had their first heat at about the same time last year in November.
Intensive research made me find this paper on the issue:
http://www.naiaonline.org/pdfs/LongT...uterInDogs.pdf
I'm going to wait one more year and will then look for the newest findings...