About the Dog Heat Cycle
The canine heat cycle — properly called the estrous cycle — is the reproductive rhythm of an intact female dog (a bitch). This calculator counts from day 0, the first day you notice bloody discharge, and projects the fertile breeding window (days 9–13), the estimated ovulation day (about day 11), an estimated whelping date (about 63 days after ovulation), and the next expected heat based on your dog's own cycle interval.
The four phases of the cycle
- Proestrus (about 9 days): Bleeding and vulvar swelling begin. Males are attracted but the female refuses mating. This is day 0 in the calculator.
- Estrus (about 9 days): Discharge lightens to straw-colored or pinkish, and the female becomes receptive and will "flag" (hold her tail aside). This is the fertile window, roughly days 9–13 from the start of bleeding, and ovulation occurs within it.
- Diestrus (about 60–90 days): The receptive period ends. If pregnant, gestation runs through this phase; if not, hormone levels still mimic pregnancy.
- Anestrus (about 4 months): The reproductive rest period between cycles before proestrus begins again.
How often dogs cycle
Most dogs come into heat about every 6 months — roughly twice a year — but the interval is breed- and individual-dependent. Small breeds often cycle every 4–6 months; large and giant breeds may cycle only every 8–12 months. A bitch's first heat usually arrives between 6 and 12 months of age, later in large breeds (sometimes up to 18–24 months). Because averages hide wide variation, the calculator asks for your dog's interval: her own history is the best predictor of her next heat.
Finding the fertile window
Peak fertility falls in estrus, typically days 9–13 counted from the first day of bleeding, with ovulation around day 11. After ovulation the eggs need about 2 days to mature before they can be fertilized, and canine sperm can survive several days in the female tract, which widens the effective breeding window. Day-counting alone is only an estimate — progesterone blood testing and vaginal cytology at your veterinarian pinpoint ovulation far more precisely and are strongly recommended before a planned breeding.
Gestation and whelping
Canine gestation averages about 63 days (9 weeks) from ovulation. Counted from the mating date it appears to range from 58 to 68 days, mostly because sperm can fertilize eggs across several days. The calculator estimates whelping as 63 days after the predicted ovulation day.