Female dogs have their first estrus cycle -- known as coming on heat or being in season -- at around 6 to 7 months, though it can be as early as 4 months or as late as 2 years in some breeds and individuals. From then on, most bitches come on heat twice a year. During estrus, female dogs have a blood-stained discharge. Pro-Estrus Bleeding The first stage of estrus is pro-estrus, lasting 4 to 15 days, and a hormone called estrogen is produced causing changes in the bitches' wombs and vaginas in preparation for mating.