I just have to say that I'm SO worried about this little girl!
I've been thinking about this ever since the OP, and I just feel so bad for her...
TinyBit, I know she is your dog and you do have the right to do whatever you want, but PLEASE PLEASE read all this advice...most of these posts AREN'T meant to offend you, only help you and your little girl. You DID ask what the dangers were, am I right? I hope you're not taking offense to all of this, and I hope you aren't just rolling your eyes at everyone's comments (I am not accusing you of anything) or skipping through people who don't agree with you.
Your little girl really needs to be spayed, ESPECIALLY at her age. Please google something about small dogs being bred too early and all the issues it can cause. She is STILL a baby herself!! You're asking for knowledge and opinions, so please girl, consider everything you're being told!! Many of these breeders here have been doing this a LONG time, and have had many tragedies throughout...and even I, a young adult, have heard of several horror stories that start out like this! My little Sophie is 3 1/2 lbs at 8 months, and she is just NOT built to have puppies. 3 lb dogs should NEVER have been bred, but unfortunately it runs rampant these days...but you should know that it is IRRESPONSIBLE! If you REALLY want to breed, please terminate this pregnancy and get your TINY girl spayed, and then talk to some reputable breeders here on YT or in your area that are willing to sell you a standard size female (5-7 lbs) who will be big enough to breed with your male.
Sweetie, we are NOT trying to be hateful, only trying to save the life of your little girl, your BABY whom you've had since she was only a few months old, from having horrible complications and/or dying during a birth that should NEVER happen! I'm sorry if ANY of this has come across as ugly or mean, that's not my intention. I just want you to REALLY think about this before you go through with it. If you want to talk, feel free to PM me anytime!