Signs of being 'in heat' include swelling of the external female genitalia and vaginal bleeding. The onset of bleeding is considered the first day of heat. Female dogs usually bleed small amounts for a week or so. The discharge then shifts from bloody to clear and then stops completely. Ovulation occurs on days nine to 11 in most dogs.
Female dogs come into heat every six months or so.
Everything is individual, just like women's cycles. Repeat breeding twice daily during these days may yield higher chances of a pregnancy. Bitches ought to be tested long before the heat for diseases such as Brucellosis, eye defects, Hemophilia and hip dysplasia. They should also be fully vaccinated and dewormed. Hip x-rays are best done at 18 to 24 months of age.