Porcheesy...it is unwise to spay a large breed dog before she is 18 to 24 months old. This means, as a result, that you will go through three or four heats, and it can be a big burden, BUT, can you afford the health consequences of spaying her early? AND, there are consequences for her, her bones haven't finished growing and the incidence of bone-related medical issues is HUGE!
SAVE YOURSELF THE POTENTIAL PROBLEMS, Save Your Pup The Pain and Arthritis, AND, Personally, I believe that if you remove her hormones early, her mental development will suffer. Now, I don't know how many people will back me up on the mental development, but, again, PERSONALLY, I HOLD THIS OPINION.
Most people here will warn you to WAIT. Wait the 18 to 24 months and let her bones, hips, knees, finish growing and becoming strong, and then at 24 months, have her spayed a month before or a month after her heat. Again, letting the hormones settle down before a spay procedure.
And, I know, your Vet will be telling you all sorts of nonsense about doing it early to prevent cancer, etc., BUT, it's a lie.
Large Breed Dogs Need To Grow Into Their Bones!
Best of luck!