The Four Stages of the Canine Heat Cycle
The canine reproductive cycle, formally called the estrous cycle, has four distinct stages. Understanding each stage helps owners identify where their dog is in the cycle, when she is and is not fertile, and when veterinary intervention may be appropriate for intentional breeding or unwanted pregnancy prevention.
Stage 1: Proestrus (Days 1-9)
Proestrus begins with the first sign of bloody vaginal discharge and vulvar swelling. This stage lasts an average of 9 days, though it can range from 3-17 days. During proestrus, male dogs are attracted to the female, but she will actively refuse mating. She is not yet in the fertile window. Owners monitoring for heat should note the first day of discharge as Day 1.
Stage 2: Estrus (Days 9-18)
Estrus is the fertile phase. The discharge lightens from bloody red to a straw or pinkish color, and the female becomes receptive to mating. Ovulation typically occurs between days 9-11 of the cycle (counting from Day 1 of proestrus). However, the eggs require 48-72 hours to mature after ovulation before they can be fertilized.
The actual fertile window is approximately days 11-15, accounting for egg maturation time and sperm survival in the reproductive tract (up to 5-7 days). The total estrus phase lasts approximately 9 days on average.
Stage 3: Diestrus (Days 18-80)
Diestrus begins when the female stops accepting males. This phase lasts approximately 58-63 days regardless of whether the dog is pregnant. Progesterone remains elevated throughout. In intact females, a false pregnancy can develop during diestrus, causing mammary gland development and nesting behavior without actual pregnancy.
Stage 4: Anestrus (Months 3-5)
Anestrus is the inactive period between cycles. The reproductive system is at rest, with low hormone levels. Most dogs cycle every 6-12 months; small breeds tend toward 6-7 month intervals, while giant breeds may cycle only once per year.
Accurate Timing for Breeding: Progesterone Testing
Calendar tracking has significant individual variation and is unreliable for pinpointing the optimal breeding day. Serum progesterone testing is the gold standard. Levels rise sharply at the LH surge (ovulation trigger) from a baseline of 1-2 ng/mL to above 5 ng/mL at ovulation. The optimal breeding window is when progesterone is between 5-25 ng/mL. Testing every 1-2 days during estrus provides the precision needed for planned breeding, especially when using chilled or frozen semen.
Preventing Unwanted Pregnancies
Intact females in heat should be kept completely separated from intact males for the duration of both proestrus and estrus (approximately 18-21 days). Males can detect a female in heat from significant distances. A single accidental mating during the fertile window will almost certainly result in pregnancy. Emergency contraception (mismate injection) is available from veterinarians if mating occurs.