Understanding Your Dog's Reproductive Cycle
If you own an intact female dog, the heat cycle — formally known as the oestrus cycle — is something you will encounter regularly throughout her life. It can be bewildering the first time: a swollen vulva, unusual bleeding, behaviour changes, and suddenly every male dog in the neighbourhood seems unusually interested. Understanding what is happening physiologically, and what to expect at each stage, makes the whole experience far more manageable.
How Often Does a Dog Come Into Heat?
Most female dogs cycle twice a year, roughly every six months, though this varies considerably by breed and individual. Small breeds may cycle more frequently — up to three or even four times annually. Giant breeds such as Irish Wolfhounds or Great Danes may cycle as infrequently as once a year. Irregular cycles, particularly in young dogs, are common and generally not cause for concern in the first year or two.
The full reproductive cycle is called the oestrous cycle and is divided into four distinct phases, each with different hormonal drivers and clinical signs.
The Four Stages of the Oestrous Cycle
Proestrus: The Beginning
Proestrus is the opening phase, lasting an average of nine days, though anything from three to seventeen days falls within normal range. During proestrus, oestrogen levels begin to rise, stimulating the development of follicles on the ovaries. The visible signs are hard to miss:
- Vulval swelling, which can become quite pronounced
- A bloody vaginal discharge, which varies from bright red to straw-coloured
- Increased attention from male dogs, though the female is not yet receptive to mating
- Behavioural changes including restlessness, increased urination, and sometimes clinginess or irritability
Importantly, despite attracting males, your dog will actively reject mounting attempts during proestrus. She may sit down, snap, or move away. This is normal — she is not yet fertile.
Oestrus: The Fertile Window
Oestrus is the phase during which your dog is fertile and willing to mate. It typically lasts five to nine days, though again there is individual variation. As oestrogen levels peak and then drop, there is a surge in luteinising hormone (LH) that triggers ovulation — usually occurring one to two days after the LH surge. Eggs require a further 48 to 72 hours to mature before they can be fertilised, meaning the peak fertile window is roughly between days three and seven of oestrus.
During oestrus:
- Vaginal discharge typically lightens in colour, becoming more straw-coloured or pinkish
- Vulval swelling may soften slightly
- The dog becomes receptive to mating — she may stand and hold her tail to one side (flagging) when approached by a male
- She may actively seek out male dogs and attempt to escape gardens or leads
If you are not intending to breed, this is the period of highest risk, and intact males should be kept entirely separate — through fences, separate rooms, or supervision on lead at all times. Male dogs can be remarkably persistent and creative in their efforts to gain access.
Dioestrus: The Post-Ovulation Phase
Whether or not mating has occurred, dioestrus follows oestrus and is dominated by progesterone. This phase lasts approximately 60 to 90 days and is, hormonally, almost identical in pregnant and non-pregnant dogs. The body prepares for potential pregnancy regardless. It is during dioestrus that false pregnancy symptoms may emerge towards the end of the phase, as progesterone falls and prolactin rises.
Visible signs return towards normal during dioestrus — discharge ceases, vulval swelling resolves, and male interest wanes. Most dogs return to their usual behaviour relatively quickly, though some remain somewhat unsettled for a week or two.
Anoestrus: The Resting Phase
Anoestrus is the period of reproductive inactivity between cycles, typically lasting four to five months in dogs that cycle twice yearly. Hormonally, this is a quiet period — no visible signs, no attraction of males, no behavioural changes related to reproduction. The body is simply preparing for the next cycle.
When Does a Dog First Come Into Heat?
Most dogs have their first heat between six and twelve months of age, though giant breeds may not cycle until 18 to 24 months. The first few cycles are often irregular and may produce less obvious signs. It is not uncommon for a first heat to be described as a "silent heat" — the hormonal changes occur but physical signs are minimal or missed.
Conversely, early first heats can occasionally occur in small breeds from as young as four months. Any vulval swelling or discharge in a young puppy warrants veterinary assessment to rule out infection or precocious puberty.
Managing a Dog in Heat
Practical management during a heat cycle requires attention and some planning:
- Keep her on lead in public spaces and secure all garden exits — even a momentary lapse can result in an unplanned mating
- Avoid dog parks and areas frequented by intact males during proestrus and oestrus
- Use washable dog pants or sanitary pads to manage discharge indoors if needed
- Be aware that female dogs in heat can provoke significant behaviour changes in intact males living in the same household, including fighting between males and persistent mounting attempts
- Expect some behavioural changes in your dog herself — patience and routine are helpful
When to Consult a Vet
Most heat cycles are uncomplicated, but veterinary advice is worth seeking if cycles are absent by 24 months, if a cycle lasts longer than four weeks, if discharge is heavy, foul-smelling, or green, or if your dog appears unwell at any point during the cycle. These can be signs of infection, hormonal imbalance, or — in the weeks after oestrus — the early stages of pyometra.
A dog's reproductive cycle is a normal part of her biology, but being well-informed about what each phase looks like makes it far easier to keep her safe, comfortable, and healthy throughout.