The corpus luteum is a hormone secreting part of the female reproductive tract. It is formed on the ovary, at the site where a mature egg cell has been released. It produces estrogen, and more importantly for the pregnancy, progesterone, a hormone that prepares the uterine lining so that a fertilized egg cell can implant, and continue to grow and develop. When the egg is not fertilized, the corpus luteum degrades after a week or two. For a fertilized egg, corpus luteum must successfully produce progesterone or the fertilized egg cell may be lost, a menstrual period may begin and the pregnancy end.
If it is known that this is happening, a woman can be given supplemental progesterone either by injection, orally, or vaginally to try to help the pregnancy proceed.