Non-spayed female dogs will go into “heat” or estrus usually twice a year. The age at which they start their cycles and the duration of the cycle varies greatly between the breeds of dogs and individual dogs
There are four stages to the canine estrus cycle:
Proestrus: vaginal discharge, males attracted to females, females unwilling to mate. Length: 4-20 days.
Estrus: swollen vulva, yellowish vaginal discharge, mating occurs during this phase. Length: 5-13 days.
Metestrus (or Diestrus): period after estrus or mating. Length: 60-90 days. If pregnant, pregnancy lasts between 60-64 days in the dog.
Anestrus: period of inactivity (sexual and hormonal) between estrus phases. Length: 2-3 months.
Some general “rules of thumb” for canine estrus:
The first estrus cycle usually occurs by age 6-12 months; for some small breeds, as early as 5 months, and for some large and giant breeds, the first cycle may not occur until 14 months of age or older.
On average, dogs have two cycles a year.
The estrus cycle lasts on average 12-21 days, but maybe be as short as a few days to four weeks. The estrus period length varies widely between breeds and individual dogs.
The length of a cycle varies widely, even for dogs of the same breed. If in doubt, assume the longer end of the range for the cycle length.
Bleeding occurs prior to a female being receptive to a male (allowing mounting by the male), but male dogs will be very attracted to the female in the proestrus stage.
Dogs can get pregnant during their first heat cycle, but this is not advisable as a 6-month old dog is not yet fully grown/mature, and complications for the mother and the puppies are more likely.
For dogs that will be pets, it is recommended to spay them before the first heat, eliminating the risk of accidental pregnancy and reproductive diseases later on in life. Dogs may be spayed while in heat (or pregnant), but there is additional risk due to the engorged vessels and tissue of the reproductive tract — a higher chance of bleeding during surgery or other complications. The cost of surgery while in heat or pregnant is often higher as well.