The canine estrous cycle
Most female dogs that have not been spayed come into heat (estrus) about twice a year, every 6 months on average. The interval varies between dogs and breeds, from about 4 months in some small breeds to 12 months in some giant breeds and in Basenjis, which cycle once a year. The first heat usually comes between 6 and 15 months of age, later in large breeds.
Stages of a heat
| Stage | Typical length | Signs |
|---|---|---|
| Proestrus | about 9 days (3 to 17) | swollen vulva, bloody discharge, males attracted but rejected |
| Estrus | about 9 days (3 to 21) | discharge often lighter, the female accepts mating |
| Diestrus | about 2 months | the cycle ends, pregnant or not |
| Anestrus | about 4 months | resting phase before the next heat |
The fertile days usually fall about 9 to 14 days after the first sign of bleeding, but ovulation can be much earlier or later. Breeders use blood progesterone tests, or vaginal smears, to time mating accurately.
How the calculator works
From the first day of the last heat and the usual interval, it estimates:
- The fertile window of that heat: about days 9 to 14
- The expected end of the heat: about day 21
- The start of the next heat and the one after
Keep a record of each heat for a year or two to learn your dog’s own interval and make the estimates more accurate.
Example
If your dog’s last heat started on October 5 and her interval is 6 months, her fertile days were around October 14 to 19 and her next heat should start in early April.
Practical tips
- Keep her on a leash and away from intact males for the whole heat, about three weeks.
- Washable dog diapers help with discharge indoors.
- Signs of pyometra, a uterine infection that often appears 4 to 8 weeks after a heat (drinking more, lethargy, discharge, swollen belly), need urgent veterinary care.
- Discuss spaying with your vet if you do not plan to breed: it prevents unwanted litters and pyometra and lowers the risk of mammary tumors when done early.