Answer
At ovulation the ovum/egg is released and the remaining follicular structure collapse inwardly to form the corpus luteum. Under the influence of luteinizing hormone (LH), this yellow body is organized and begins to secrete both estrogen and progesterone, but predominantly progesterone. However, the level of LH soon declines and the CL dies soon if fertilization of the ovum does not occur.
Work Step by Step
When fertilization occurs-- about 18 hours after ovulation--the fertilized egg develops into a blastocyst. Initially it is the secretion of human chorionic gonadotropin (hCG/HCG) by the blastocyst that sustains the CL and keeps it from disintegrating. Implantation of the blastocyst into the endometrium occurs about 10 days after ovulation.The CL has many functions, but the most important one is to secrete progesterone, This progesterone begins preparing the uterine endometrium to receive the blastocyst at implantation. Progesterone also also keeps the myometrium of the uterine quiet so that it does not contract and expel the blastocyst; that would end the pregnancy. These functions of the CL are essential during the first few weeks of pregnancy. However, the placenta and the pituitary soon take over the duties of maintaining the pregnancy.
