The female dog reaches puberty at a somewhat variable age depending on the size, although on average the pubertal age can be fixed at around 9 months.
From then on, sexual activity will go on for life as there is no menopause. The bitch is a spontaneously ovulating animal, regardless of mating.
It also has only one heat (also called estrus) in which the female is available for mating. The bitch's cycle is mono-monthly and not seasonal, as estrus occurs regardless of the season. On average there are two cycles per year with significant variations according to the sizes. When the bitch reaches puberty, the sexual cycle will start, divided into 4 phases:
Proestro : in this phase the heat begins. The period lasts 7-10 days and the female attracts males but refuses to mate. Estrogen, female hormones increase the peak;
Estro : during this period of variable duration from 3 to 21 days, the bitch accepts the male and is ready to mate. At the hormonal level, there is a peak of the hormone LH, which stimulates ovulation which occurs 1 or 2 days after the start of estrus. During this period the female can be fertilized by several males.
Diestro : it lasts an average of 90 days reaching even 130-140. This stage occurs when the bitch is not pregnant;
Anestro : is the period of sexual inactivity and lasts for 2-3 months. The female has no mating instinct and does not like being mounted by the male.
The cycle length is over six months.