This stage may last anywhere from 4 to 20 days. You may see some blood or vaginal discharge. Your female will be unwilling to mate, but males will be very interested. Estrogen production increases during this time as the uterus enlarges and prepares for fertilization. Increase in estrogen causes the vaginal walls to take on a distinctive shape, known as cornification. Measuring levels of estrogen as well as checking for cornification can be good indicators that the heat cycle has begun.
This stage lasts 5 to 13 days, and mating will occur at this time. The vulva is swollen, and there may be yellowish discharge. Increased estrogen production gives way to increased progesterone production as the body prepares to nourish the eggs once fertilized. Measuring progesterone can be a good indicator that your female is ready to mate, but behavioral changes will also be easy to spot. Females will now be seeking out the males to encourage mating by giving clear signals such as backing toward the male and wagging her tail in wide swoops to alert males to her scent.
This is the period after mating when, again, the females are not willing to mate. If your female is pregnant, the pregnancy will last 60 to 64 days. Otherwise, the length of this stage is 60 to 90 days. During this stage, progesterone, which is produced to nourish the eggs and help them grow into fetuses, may cause “false pregnancy” symptoms such as nesting behavior and milking, even if the female is not pregnant. This phase ends when progesterone levels have returned to normal either after whelping new puppies or after an approximately two month period for non-pregnant females.
The 2 to 3 month period of inactivity in between heat cycles.
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.