fertile time in heat?

Please tell me you are a troll. The puppies will probably die as the mother is unlikely to survive the whelping, so you won't need to worry about whether they are long or short coated.
There is no such thing as a tea cup Shih Tzu.
References :

Oh my gosh.

MY IGNORANT BACKYARD BREEDER SENSE IS TINGLING!

You don't know the first thing about breeding. PLEASE go get your dogs spayed and neutered.

EDIT: It's not a teacup Shih-Tzu. Imperial? That's a new one. You have a runt. A generations-long runt. Who is still probably way bigger than your poor Chihuahua. Those pups (THAT WILL HOPEFULLY NEVER HAPPEN) would be way too big for her to whelp.
References :

No…they have to breed two weeks after she is in heat. Never do it while she's in heat.
References :

when she's about 2weeks in she'll get pregnant..she will stop nipping him then to and if she has a long hair gene then yes they could have long hair puppies..And omg I love teacup shih tzus they are so cute..
References :

Are you aware that she'll probably need a c-section? And that if you don't get her in for one at the right time, not only will the puppies die, but she will die?

Why take the chance on killing your dog to produce more poorly bred, unhealth tested mutts?
References :

If you need to ask this question here, you don't know anywhere near enough to be breeding.

Please go do some serious research on responsible breeding.
http://learntobreed.com/
http://woodhavenlabs.com/breeding.html
http://dogplay.com/Breeding/index.html
References :