During pregnancy, when a fertilized ovum implants in the uterine wall, the corpus luteum is maintained through the actions of a non-pituitary gonadotropin, called hCG - human chorionic gonadotropin, which is secreted by the endocrine cells of the placenta. Progesterone continues to be excreted by the corpus luteum, which serves to enrich the uterus with a thick layer of blood vessels.
hCG is structurally similar to the gonadotropins produced by the pituitary, as they share the same alpha subunit with FSH and LH, as well as TSH.
