Literally translates to yellow body. Formed in the follicle after an egg is released. Produces progesterone, which causes the lining of the uterus (endometrium) to thicken, enabling a fertilized egg to burrow into the uterus. A deficiency in the amount of progesterone produced (or the length of time it is produced) by the corpus luteum can mean that the endometrium is unable to sustain a pregnancy. This is called luteal phase defect (LPD).