A female dog’s estrus cycle is divided into four stages -- proestrus, estrus, diestrus and anestrus. When people refer to a dog being in heat, this is typically the estrus cycle. During this phase, a female dog is ovulating and capable of breeding. Diestrus is the stage that follows, when the dog is no longer able to mate. During this stage, many changes occur, whether the female dog conceived or not.
A dog’s estrus cycle begins with proestrus. This stage averages nine days. You may notice vaginal bleeding or discharge, behavioral changes, appetite changes and swelling of the vulva. You may also notice her tail hangs down and she urinates frequently. After this, she progresses into estrus, the stage where mating occurs. You may notice the discharge lightens to a tan color and the vulva swelling reduces. She will fan her tail as if to sign to male dogs that she is available. Estrus lasts for an average of 9 days. Your dog enters diestrus after estrus, whether mating occurs or not. Diestrus lasts for around two months before moving into anestrus. Hormonal activity ceases during anestrus, allowing the body to prepare for another cycle.
Diestrus signals the end of the mating period, when the female is no longer receptive to the male. Vaginal discharge may return to bright red before tapering off and ending. Swelling of the vulva will go down until the vulva returns to normal. Any interest and flirtation your dog may have displayed during estrus stops.
During diestrus, hormone levels change. Estrogen levels are low while progesterone levels gradually begin to increase. Peak progesterone levels occur typically three to four weeks after the start of diestrus before gradually decreasing to baseline levels. This change in hormones occurs whether mating has occurred or not.
If breeding was successful, there will be slightly different hormonal changes during the diestrus period. After the progesterone reaches peak levels and starts to drop, prolactin levels begin to increase and peak near the end of pregnancy and diestrus. The hormone relaxin begins to increase around day 20 of pregnancy and diestrus, peaking around day 40.
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