If you are wanting to breed from your female dog, are wondering how to manage her heat period, or are just interested in dog biology, this is the article for you! Today we will be learning all about the female dog’s reproductive cycle, and answering the question ‘when do dogs go into heat?’

Let’s start with the basics.

What is a Heat?

A ‘heat’ is the period of a female dog’s oestrus cycle where she becomes receptive to a male (allows him to mate) and can become pregnant.

Hold on though, let’s go back a bit; what is an oestrus cycle? An oestrus cycle is a set hormonal, physical, cellular, physiological and behavioural changes in a female animal that prepares her body for pregnancy. The equivalent in humans is the menstrual cycle. However each mammal has a very different oestrus cycle with different timings, hormones and processes involved.

Understanding Dog Oestrus Cycles

A female dog is non-seasonal monoestrus. This means she has a complete oestrus cycle around once a year that is not tied to the seasons. Humans are non-seasonally polyoestrus; the biological females among us have multiple cycles per year, on average one every 28 days, regardless of seasons. By contrast, cats are seasonally polyoestrus; so they have multiple cycles in spring and summer, but often none in autumn and winter.

How long does a dog’s oestrus cycle last?

A dog’s complete oestrus cycle lasts around 210 days, or 7 months (though this can be very variable between individuals and breeds!). 

The Follicular Phase

The first phase is called the follicular phase, and contains pro-oestrus and oestrus periods. Day 0 is ovulation, when she releases eggs from her ovaries into the uterus. Before ovulation is pro-oestrus, as ovulation occurs during oestrus. If a female dog mates with a male dog close to when she ovulates, she can become pregnant. 

The Luteal Phase

Regardless if she is pregnant or not, a structure develops on the ovary called the corpus luteum– this takes place during the luteal phase, which is made up of metoestrus and dioestrus periods. The corpus luteum produces a hormone called progesterone that maintains the structure of the uterus. In short, progesterone creates an environment that allows eggs to develop into puppies.

The corpus luteum slowly degrades over time, taking 60-70 days, which covers the roughly 63-day pregnancy. If she is not pregnant, the corpus luteum still produces progesterone and the uterus is still prepared for egg; in other words, a false pregnancy. This is why some dogs can act pregnant after their heat, even if they are not pregnant; all the hormones in her body are telling her that she is pregnant.

After either a normal pregnancy or a false pregnancy, there is a long period where her ovaries are not active. This is called anoestrus. No eggs are produced, hormone levels are low and she cannot become pregnant. It lasts about 5 months (sometimes more). This allows her to raise any puppies that may be present, and for her uterus to recover and prepare for the next cycle. 

After anoestrus, a few weeks before her next ovulation on day zero, the follicular phase starts again, specifically pro-oestrus. Her ovaries reactivate and develop eggs, her uterus is starting to prepare for pregnancy again, and her behaviour changes – this is the start of the next heat.

When Do Dogs Go Into Heat?

So to sum up, when is heat – heat is a few weeks either side of ovulation (day zero), during the follicular phase, specifically during pro-oestrus and oestrus periods. Bear in mind that ovulation can vary from a few days to a few weeks after the oestrus period starts, so day zero can vary – this can mean knowing when to mate her for a successful pregnancy can be difficult.

All of that biology and strange terms are quite complex, so how can we identify heat more easily?

The easiest method is to look at your dog’s behaviour and appearance. When a dog is in heat, she is preparing for pregnancy. This means she will become increasingly receptive to males (will become more and more playful, and eventually stand to allow them to mount her), may be more anxious or aggressive, and may lick her genitals more. She may also urinate more frequently.

If you look at her vulva, it will be more red and swollen, and there may be some bloody or yellow discharge. The volume of discharge decreases and it becomes more colourless as she gets closer to ovulation. Her mammary glands may enlarge, and male dogs will be more interested in her. Some dogs do not show obvious heat signs, and have a ‘silent’ heat. 

Tests for being in heat

If you are wanting to more accurately identify heat, particularly if she is having a silent heat, there are a number of tests vets can perform. First, we can perform a digital exam of her vagina. The tone and thickness of her vaginal tissue will change over her cycle, and we may be able to identify discharge. Closer inspection can be performed with a speculum or endoscope, to view the cervix and determine if it is open in preparation for reproduction. 

Swabs of the vagina can be taken, and the material looked at under a microscope. Before ovulation, the cells of the vagina become larger and lose their nucleus. This can be identified by skilled vets. We sometimes call these cells ‘cornflake’ cells. We may also take a blood sample and measure hormones like progesterone. As these hormones vary so much throughout the oestrus cycle, the current level can help to identify when she is in heat. 

Finally, we can use imaging to look at your dog’s internal reproductive organs. Ultrasound is most commonly used, to look at the size and thickness of the uterus and the appearance of the ovaries. A larger thicker uterus and active ovaries indicates she is in heat and preparing for pregnancy. Radiography/x-rays can be used too, but are more commonly reserved for diagnosing pregnancy later on. 

The long wait…

Another important factor to remember is that during the anoestrus period, her ovaries will be inhibited and she will not cycle again. This anoestrus period is on average 5 months, but can last indefinitely in some situations. If you are wanting to get your dog pregnant again, you will need to wean her pups and may have to separate them from the mother. Otherwise, the mother’s next heat will usually be delayed.


You may have a puppy and be wondering when she will have her first heat. Just like all mammals, dogs will not become fertile until puberty. The point of puberty, when she first starts cycling, varies greatly between breeds. Smaller dogs can enter puberty as young as four months, while larger dogs can be a year or two old. It can simply be a waiting game with some dogs.

To make matters more confusing, their first heat is more likely to be silent, and thus missed. If you are wishing to breed from a young dog, regular exams by your vet can help identify when they are first entering puberty. If you are not wanting her to become pregnant, it can be useful to identify her first heat, so you know when to start being extra careful around male dogs and discuss spaying with your vet. 

Final Thoughts

Reproductive biology is some of the most complex science there is, but we hope this article has given you a bit more knowledge to help you identify a dog in heat. Remember that every dog is different, and individuals present in heat very differently. If you are having trouble, please ask your vet for assistance using some of the more accurate methods of heat detection. 

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