In large monkeys and humans, menstruation marks the beginning of the reproductive cycle which prepares the womb for pregnancy. However, in dogs, there is no menstrual cycle but an oestrous cycle. The oestrous cycle has 4 stages namely the Proestrus, Estrus, Diestrus and Anestrus. 

The Proestrus and Estrus are the two main stages of the cycle concerned with mating. At Proestrus, there is a swollen vulva with bloody discharge lasting for 9-10 days. The Estrus is the heat period where ovulation occurs and the bitch is receptive to the male for mating. It usually lasts 7-9 days.  

What day of the heat period should a bitch be mated? 

Count roughly 7-10 days from the day you first noticed blood (1st day of the cycle) to start mating. The easiest way to detect that your dog is already in heat is to put her together with a male, once she is ready, she will allow the male to mate with her.  For a precise fertility test, a progesterone assay combined with vaginal cytology can be done to predict the exact days of mating. Breeding based on the recommendation from a fertility test has higher success rates. 

What Age Should You Breed or Mate Your Female Dog ?

Generally, between 1 ½ – 2 years (depending on the breed of the dog), breeding can commence. This is to ensure full maturity which is safe to carry a pregnancy and nurse puppies.  A simpler way to calculate it is to mate a bitch at her 2^nd or 3^rd heat. The first heat should be skipped.  Once a bitch clocks 6 years, she should be closing out on breeding to prevent health complications. Usually, at that age, the fertility rates would have also reduced.