I was told by my vet that with larger breed dogs, if you spay or neuter when they're too young it could stunt their growth. I waited to get my lab neutered until he was a year and a half old. It cost me more money because at the clinic I go to they base it off the weight of the dog. Whether there's any truth to that I don't know. My boy is about 90-95 lbs. 17 weeks does seem a little young to me but I'm no vet either.
my vet said app. 6 months old, after the first HORMONE drop... it is much easier on the bitch then, and less long term health issues as well with spaying at that age. Done it both ways , I am a believer!