Events in the uterus and vagina over the four phases of the 21-day cycle

Pre-oestrus:
Duration: approx. 2 days
The muscles of the uterus start to contract. The cervix opens. The vaginal mucosae become moist and red. A vaginal discharge may be observed.

Oestrus:
Duration: 12-24 hours
During oestrus, the muscles of the uterus contract to the maximum. This is why the uterus is small and firm on examination. The vaginal mucosae are usually pale pink and moist, as the uterus and cervix produce a (clear, stretchy) oestral mucus. The cervix may be open, up to the diameter of a pencil. The vulva appears swollen and there is a clear, stretchy mucous discharge which is produced mainly in the cervix but also in the uterus.

Post-oestrus:
Duration: 1-2 days
The muscles of the uterus relax and the cervix slowly closes again. The vaginal mucosae are less moist. The swelling of the vulva subsides. A bloody discharge may be observed coming from the vagina. This is called metoestral bleeding. The blood comes from small blood vessels (capillaries) in the uterine musculature which were torn by the strong contraction (capillary bleeding). Bleeding is a useful indicator and can be recorded on the oestrus calendar (bleeding minus 2 days = oestrus).

Inter-oestrus/Dioestrus:
Duration: approx. 15 days
The muscles of the uterus are relaxed, so the uterus feels soft and flaccid. The vaginal mucosae are less moist and the cervix is tightly closed. The vulva appears wrinkled.