Female dog in heat: signs, duration, care and how to prevent unwanted pregnancies

What is a "season" in dogs?

A season (or oestrus) is the part of the reproductive cycle when a female dog can become pregnant. Dogs are monoestrous — typically two cycles per year, though this varies by breed and size.

When does the first season happen?

• Small breeds: 6–8 months of age.
• Medium breeds: 8–12 months.
• Large breeds: 12–18 months.
• Giant breeds (Great Dane, Newfoundland): up to 24 months.

Phases of the reproductive cycle

1. Proestrus (7–10 days): vulval swelling, bloody vaginal discharge, female attracts males but does not accept mating. Frequent licking of genitals.
2. Oestrus (5–9 days): lighter or straw-coloured discharge, female accepts mating. This is the fertile window.
3. Dioestrus (60–90 days): hormone decline if no pregnancy. Phantom pregnancy (pseudopregnancy) can occur.
4. Anoestrus: rest phase between cycles (4–5 months).

The visible season (proestrus + oestrus) lasts about 21 days on average — but can range from 2 to 4 weeks.

Managing a dog in season

• Never leave unsupervised: intact males can mate through fences or partially open doors. If you don't want a litter, supervision must be total.
• Lead walks only: avoid areas frequented by intact male dogs.
• Dog nappies / pants: protect furniture and floors from discharge.
• Mood changes: some bitches become more anxious, clingy or irritable. Normal — resolves after the cycle.

Warning signs

• Fever, foul-smelling or greenish discharge → possible pyometra (a medical emergency). Vet immediately.
• First season before 6 months or after 24 months → vet check.
• Severe phantom pregnancy (swollen mammary glands, maternal behaviour) that persists → discuss hormonal treatment with your vet.

Options for preventing unwanted pregnancies

• Spaying (ovariohysterectomy): the permanent, most recommended option. Eliminates pyometra risk, reduces mammary tumour risk, and prevents unwanted pregnancies permanently.
• Hormonal contraception: progestins by injection or tablet. Short-term use only under veterinary supervision — increases pyometra and tumour risk.
• Emergency contraception: oestrogen injection within 48h of mating. Effective but with significant side effects — only appropriate in confirmed mating situations.