Unspayed female dogs tend to experience estrus, or go into heat, twice a year. Some breeds go into heat more often than others, and spayed dogs do not go into heat.
Estrus is when a female dog begins the process of being receptive to mating with a male. The estrogen levels in the female dog increase and decrease at a quick rate, and the ovaries release mature eggs to be fertilized. Female dogs experience estrus for about 18 days, a period during which male dogs are attracted to them. However, females are responsive for only about nine days.
The first heat can occur at 6 months old for smaller breeds and as late as 24 months for larger dogs.