Sexual cycle

The sexual cycle in dogs describes how the hormones and body of a female dog change in alternation – and when she is ready to reproduce. Anyone who owns a female dog will become familiar with this topic at the latest the first time the vulva suddenly swells and a bloody discharge appears. The cycle repeats at regular intervals and proceeds in four clearly distinguishable phases that fundamentally influence the behaviour of the female dog.

Phases of the sexual cycle in dogs

Four phases form the basic framework: pro-oestrus, oestrus, di-oestrus and an-oestrus. Each has its own hormone profile – and anyone who knows them can interpret the behaviour of their female dog much better.

Pro-oestrus (pre-heat)

• Duration: Around 7 to 10 days.
• What happens: The cycle starts. The vulva swells visibly, and a bloody vaginal discharge appears – for many owners, this is the first real warning signal. Although the female dog becomes increasingly interested in male dogs, she does not yet allow herself to be mated.
• Behaviour: The female dog is often more restless than usual, sniffs more intensely and seeks contact – but will sharply reject an intrusive male dog. This is normal and nothing to worry about.
• Hormones: Oestrogen levels rise and prepare the body step by step for the fertile phase.

Oestrus (heat / receptivity)

• Duration: 5 to 14 days – a wide range that varies depending on the female dog.
• What happens: The female dog is now receptive. The discharge becomes lighter, watery or slightly pink – a reliable sign. The vulva remains swollen. This is the actual fertile phase.
• Behaviour: Classic mating behaviour: when a male dog approaches, the female dog stands calmly with her tail slightly pushed to the side. She actively flirts, is attentive and no longer avoids the male dog.
• Hormones: Oestrogen drops, progesterone begins to rise. Ovulation usually occurs within 48 hours of the start of oestrus.

Di-oestrus (post-heat)

• Duration: Approximately 60 to 90 days.
• What happens: The heat is over – regardless of whether the female dog was mated or not. The discharge stops, and the vulva returns to normal.
• Behaviour: No more interest in male dogs, no heat behaviour. The female dog returns to her usual routine.
• Hormones: Progesterone remains elevated to support a potential pregnancy. Even without mating, false pregnancy can occur – the body reacts to the hormone signal as if fertilisation had taken place.

An-oestrus (resting phase)

• Duration: 4 to 6 months, sometimes significantly longer depending on the breed and individual.
• What happens: The reproductive system is inactive. No discharge, no swelling, no noticeable interest in male dogs. The female dog simply acts as usual.
• Hormones: Oestrogen and progesterone are at a low level. The body recovers and quietly prepares for the next cycle.

Sexual cycle in male dogs

Male dogs function quite differently. They do not have a fixed cycle – they are capable of reproduction all year round. What sets them on edge is the pheromones of a female dog in heat. As soon as a female dog is in oestrus, male dogs nearby react with increased attention, restlessness and pronounced mating behaviour. This explains why an intact male dog near a female dog in heat can hardly keep still.

Frequency of the sexual cycle

How often a female dog comes into heat depends largely on the breed. As a rough rule of thumb: once or twice a year. But the range is considerable.

• Smaller breeds: Heat every 4 to 6 months.
• Larger breeds: Often only once a year, some even less frequently.

If you have a young female dog, you should know: the first cycle usually begins between the 6th and 12th month of life, in large breeds sometimes not until 18 to 24 months.

Signs of the sexual cycle in dogs

The most important signs, especially during pro-oestrus and oestrus, can be observed well – you just need to know what to look for:

1. Swelling of the vulva: One of the first and most noticeable signals. Often clearly visible, even to untrained eyes.
2. Vaginal discharge: Bloody at the beginning, lighter or watery-pink during oestrus. The change in colour is an important indication of the transition to the fertile phase.
3. Changed behaviour: More sniffing, increased interest in male dogs, sometimes restlessness or unusual clinginess towards the owner.
4. Mating behaviour: During oestrus, the female dog actively accepts mating and shows the typical standing tolerance posture.

Reproduction and mating

For targeted breeding, timing is crucial. The optimal mating time usually lies 10 to 14 days after the start of pro-oestrus – i.e. in the middle of oestrus, when ovulation occurs. If you want to be on the safe side, have the progesterone level measured by the vet. This is particularly useful for planned matings, as the fertile phase can vary considerably from one female dog to another.

• Pregnancy: If the female dog is successfully mated, pregnancy lasts around 63 days.

False pregnancy

A phenomenon that surprises many owners: pseudopregnancy. It occurs during diestrus – even if the female dog was not mated. The body behaves hormonally as if fertilisation had taken place. The female dog may develop swollen teats, start producing milk and begin to build a nest or “adopt” soft toys. Typically, this subsides on its own after a few weeks. If it persists longer or the female dog is visibly suffering, a vet visit is advisable.

Castration and Sterilisation

Castration is the safest way to permanently end the sexual cycle and rule out unwanted pregnancies. In this procedure, the ovaries – and depending on the method, also the uterus – are surgically removed. The result: no more hormone production, no more oestrus.

In addition, castration reduces the risk of uterine inflammation (pyometra) and lowers the likelihood of certain hormone-dependent tumours, including mammary tumours. Whether and when castration is advisable should be discussed with your trusted vet – the answer depends on the breed, age and individual health status of the female dog.