I would definately be calling my vet and asking these questions. How soon can you tell if the female is pregnant, ect. And I would take every precaution possible in proctection my very tiny female from getting pregnant. I agree that it is dangerous for the female, and could cost her , her life, being so tiny and young to get pregnant.
My Suzi is 9 months old and about 8 lbs. She is not spayed yet, and I am not planning on having her have puppies. She is going to be fixed in the spring before her 1rst birthday.
I do not want to gamble my girl's health and put her at risk of all kinds of complications. Even though she is a larger Yorkie, It still scares me, and it is not a chance I am willing to take.