Ovary in Inter-oestrus

The yellow body (corpus luteum) produces the pregnancy hormone (progesterone). This hormone is transported via the bloodstream, causing changes in behaviour and in the reproductive tract. The immature yellow body is also called the corpus haemorrhagicum. It is soft, and red in colour. A mature yellow body during mid-cycle is sometimes described as "blooming like a flower". It is often shaped like a champagne cork. It should take up at least 50% of the ovary.

One or two follicles may develop on the ovary during the yellow body phase. These are referred to as follicle waves or intermediate follicles. These follicles die without ovulation taking place. The yellow body does not regress until the hormone prostaglandin is produced by the non-pregnant uterus.