Because of the size of Great Danes, they typically don't experience their first heat until they are around two years old, and they have a heat cycle every 12 to 18 months. Smaller dogs can have two heat cycles per year.
Heat cycles typically last from two to three weeks. A female is said to be in heat at the first sign of vaginal discharge. You can tell this is happening because the dog will lick her vulva and it will appear swollen. Bleeding becomes apparent a few days after the vulva becomes swollen.
If mating is desired, it may be difficult to determine when the female is most fertile. Your veterinarian can perform tests such as a serum progesterone test or a vaginal smear test that can predict ovulation.
It is possible to have a female dog spayed or desexed during her heat cycle, but it is usually not recommended and some vets will refuse to perform the procedure. During a female's heat cycle, there is increased blood flow to the ovaries and uterus, making the operation risky. It is often better to wait for four to six weeks after the end of the heat cycle to have the dog spayed.
Great Danes can bleed a lot during their heat cycles. To keep the blood off your floors and furniture, you can find doggy diapers at large pet stores or use adult diapers with a hole cut in the back to accommodate the dog's tail.