Female dogs do not menstruate in the way that women do. Instead, dogs have an estrous cycle, more commonly called a heat cycle. Sexually mature female dogs go through heat twice, or less commonly once, every year. This usually starts between 9 and 12 months of age, although this varies. Females have heat cycles throughout their lives because, unlike primates, dogs don’t go into menopause as they age. Instead, their heat cycles gradually stretch out in length. A dog’s heat cycle consists of four stages: proestrus, estrus, diestrus and anestrus. Different physical and hormonal events happen during each stage. Three hormones are essential to a dog’s heat cycle: 1) estrogen, responsible for starting the heat cycle; 2) luteinizing hormone (LH), responsible for causing ovulation; and 3) progesterone, responsible for maintaining pregnancy.