A Dog’s Menstrual (Heat) Cycle
While dogs may seem to have a menstrual cycle that is very similar to a woman’s menstrual cycle, dogs do not experience an actual menstrual cycle. Primates are the only mammals that have a true menstrual cycle. Instead, dogs have what is called an estrus cycle.
The Female Dog's Heat Cycle
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.
The first phase of a dog’s heat cycle is proestrus. On average, it lasts about 9 days but can range from 3 to 17 days in length. The dog’s vulva gradually becomes swollen and excretes a reddish-brown discharge. During proestrus, the female’s body is preparing itself for a possible pregnancy. However, she will not accept a male. Estrogen levels start rising about 1 month before proestrus starts and then peak and drop late in this phase.
The second phase of the heat cycle is estrus – not to be confused with “estrous,” which refers to the entire cycle. Estrus lasts about 9 days but can span from 3 to 21 days. During this stage, the female has a soft, swollen vulva and variable reddish, pinkish or straw-colored vaginal discharge. When estrogen levels fall in late proestrus, progesterone levels start to rise and plateau at labor or with a false pregnancy during estrus. LH levels spike sharply when estrogen levels fall and progesterone levels rise. The LH surge stimulates ovulation within 2 to 3 days, and the female becomes receptive to males. Most but not all females are amusingly flirtatious during estrus, poking the male with their nose, play-bowing, pushing their rear end into his chest and lifting their tail to one side, called “flagging.” This is called being in “standing heat.” It takes eggs 2 to 3 days to mature after they have been released from the ovaries before they can be fertilized. Progesterone must remain elevated to sustain a normal pregnancy.
Diestrus is the third phase of the heat cycle. It can range from 50 to 80 days, averaging about 60 days in length. Diestrus will begin regardless of whether the female has been bred or is pregnant. During this period, her body acts like it is pregnant. She may enter a false pregnancy. Females that aren’t pregnant may start to “mother” other animals and/or toys during this time, but they will not accept a male.
Anestrus is the fourth and last phase of the heat cycle. It can last from 130 to 250 days. Some breeds, including the Dingo and the Basenji, have very long anestrus periods and only go through one heat cycle annually. During anestrus, females are not sexually active. This is a resting phase, where the body starts to prepare itself for another heat cycle.