You must be very, very, very careful when she is in heat! This happened to me about 15 yrs. ago, and I have learned alot since :
My AKC Chi came into heat when she was almost a yr. I allowed her to go in the back yard with our female Bull Terrier, who had been fixed before we got her ...so I had no experience with dogs in heat. She was wearing panties. I left them alone for no more than 5 min when I heard a terrible ruckus. I ran outside to find our Lab hurt and bleeding. The mutt next door, (gee, we had lived there for 5 yrs and I had never even heard this dog bark ... didnt even know it was there) had knocked down the redwood fence to get at her. It had mauled our BT, who was trying to protect the little Chi. And of course, by the time I got to her ... my little Chi was hung-up with that mutt. There was nothing I could do, it happened so fast. You cannot leave her unsupervised for 1 second unless she is locked in a kennel, seriously. Had to take the BT the vet for stitches ... $$$ Then had to wait a few a weeks to see if she was pregnant .... Of course, she was. Vet heavily suggested a doogie abortion ... said the other dog was too big. She would probably die from complications. So we did it, and with all the trauma went ahead and her fixed... $$$. It was much harder on her than just getting her fixed from the begining. The whole experience was very traumtic - and expensive, not to mention the cost of fixing the fence ...$$$, and fighting with the neighbors continued for years. You have to be completely diligent ... they will find a way to get together. You cannot be too careful.