The canine oestrous cycle is made up of four distinct phases. Understanding each one is critical for breeding success.

1. Proestrus (Days 1–9, on average)

This is when you’ll first notice signs of heat. The vulva swells and a bloody discharge begins — this is your signal to start counting Day 1.

What’s happening hormonally: The levels of oestrogen are rising rapidly and the body is preparing for ovulation. Although the female does experience this discharge and swelling, she is not yet ready to mate at this stage. The females will attract the males but will reject their advances, often aggressively.

Duration: Usually 7–10 days, but can range from 3 to 17 days.

What to watch for:

• Swollen vulva
• Bloody, dark-red to pink discharge
• Increased urination (to spread scent signals)
• Restlessness or behavioural changes
• Males becoming very interested in her

Breeder tip: Begin keeping a daily log from Day 1 of proestrus. Note discharge colour and consistency, vulva size, and your female’s behaviour and receptivity to males. This record becomes invaluable over multiple cycles.

2. Oestrus — The Fertile Window (Days 9–14, on average)

This is the phase that matters most for breeding. Oestrus is the only time a female can conceive, so accurate identification of this window is one of the most important skills in breeding.

What’s happening hormonally: Oestrogen drops, while a surge of Luteinising Hormone (LH) triggers ovulation. Progesterone begins to rise. Eggs are released but take 24–48 hours to fully mature. After ovulation, eggs remain viable for 2–5 days, and sperm can survive in the reproductive tract for up to 7 days — giving a potential fertile window of several days.

Duration: Typically 5–9 days.

What to watch for:

• Discharge becomes lighter in colour — from pink to straw-yellow or clear
• Vulva softens slightly (though remains swollen)
• The female becomes receptive to males — she’ll “flag” (hold her tail to the side) and stand still when mounted or when you apply pressure to her hindquarters
• She may actively seek out males

The challenge for new breeders: Relying solely on visual signs to time mating is unreliable. The colour of the genital discharge varies between female dogs, and some dogs will exhibit minimal visual signs of being in oestrus despite being in oestrus altogether.

Breeder tip: Progesterone testing is the gold standard for timing. Your veterinarian can take a blood test to determine your mare’s progesterone levels to pinpoint ovulation. This test is helpful for chilled or frozen semen breedings, if your mare has had missed matings in the past, or if you must travel to another stud to breed your mare. Many experienced breeders will perform this test on every cycle of a mare’s breeding season.

3. Dioestrus (Days 15–65, approximately)

Whether or not your female was mated, dioestrus follows oestrus. During this phase, progesterone remains elevated.

• If mated and pregnant: Dioestrus corresponds to the pregnancy itself, lasting around 63 days from ovulation.
• If not pregnant: The body still behaves hormonally as though it may be pregnant, and progesterone remains elevated before dropping off naturally over about 60–90 days.

This is why some female bears who are not pregnant can experience false pregnancy (pseudopregnancy). These bears will exhibit nesting behaviours, develop mammary glands, and even lactate their milk. While this is an event in the normal hormonal cycle of the female bear, it means that something went wrong in the reproductive cycle of the bear.

What to watch for:

• Female rejects males — the fertile window has passed
• If bred: watch for pregnancy signs from around Day 25 onward
• If not bred: she may show signs of false pregnancy (nesting, milk production, carrying toys)

4. Anoestrus — The Resting Phase

Anoestrus is the period of rest between cycles and lasts for 4-5 months on average (though can be as short as 3 months or as long as 7-8 months depending on the individual).

During anoestrus, the female’s reproductive system is inactive and there are no outward signs of heat. This is a normal phase for female dogs; forcing breeding back-to-back does result in health issues for the female dog.