Terrier breeding

Question
I was wondering how old a cairn terrier female has to be before she comes in heat. I have a little one that's three months old & my brother has a two year old pom & he keeps trying too breed with her. I don't want her to have pups from him but i thought she was too young and she loves to play with him cause he's a little smaller than her. If u can please let me know something so I can prevent anything like that from happening.

Answer
3 months old is too young to come in season.  From about 6 months to 18 months is when they will have their first heat.  Why not get the male neutered and when the Cairn is old enough have her spayed then there will be NO accidents.  Hate to say it but a Cairn-Pom mix won't be pretty!  
The Pom is just being a boy....


Hope this helps.
Tammy