With each heat cycle that your dog has before she is spayed her chance of getting mammary cancer increases. Because of this, they recommend spaying before the first heat cycle. Some people believe that they look more mature later in life if they are allowed to go through a cycle or two before being spayed. If you are going to spay your dog then do it before, it's just better that way, plus you won't have to put up with her bleeding all over your home or attracting male dogs. After the first heat her vulva will become larger and so will her mammary glands...this will not happen if she is spayed before hand.