The corpus luteum is a yellowish collection of cells that develops on the surface of the ovary at the point where an ovum (egg) is released at the middle of a woman’s normal menstrual cycle. The corpus luteum grows to one or two centimetres in diameter, and if a pregnancy occurs, may increase to three centimetres. It produces the hormone progesterone, which nurtures the lining of the uterus (the endometrium) so that it is suitable for the implantation of a fertilised egg (zygote). After implantation the corpus luteum continues to grow slowly until three months of pregnancy, then slowly degenerates, and the amount of progesterone it produces decreases, until it disappears at about the sixth month of pregnancy.
If no pregnancy occurs, the corpus luteum rapidly degenerates after about ten days, progesterone levels drop, and a menstrual period occurs 14 days after ovulation.