Yellow body, or corpus luteum, is the name given to the yellow mass of cells that collect in the follicle of the ovary once a mature egg has been released.
The purpose of yellow body is to alert the body to the fact that fertility is at its optimum and that preparations need to be made to accommodate a possible pregnancy. Yellow body does this by secreting both oestrogen and progesterone.
Oestrogen is the hormone responsible for alerting the uterus to prepare for a fertilised egg. It stimulates the uterus into producing a thick and spongy lining of mucus. This lining will accept and cushion the fertilised egg.
Progesterone is released by yellow body in order to prepare both the cervix and uterus for pregnancy. Much like oestrogen, progesterone also has an effect on the lining of the uterus, and causes it to thicken in order to accept a fertilised egg. Progesterone also changes cervical mucus, causing it to become so thick that sperm is no longer able to pass through the cervix. This measure protects an egg that has been fertilised.
If a pregnancy has occurred, yellow body will increase and remain in place for several months. If a pregnancy has not happened, yellow body will degenerate and shrink, and will reoccur in the next cycle.