The first physical changes seen are swelling of the breasts and enlargement of the vulva. Soon to follow is a blood tinged vulvular discharge that lasts about 10 days.
The full heat cycle lasts for 3 weeks and it is possible for a dog to get pregnant anytime during the cycle. These heat cycles routinely happen about twice a year.
If you know you don’t want puppies and have no desire to deal with a bunch of glary-eyed male dogs around your house for 3 weeks – do the smart thing and have your dog spayed before 6 months of age.