Human chorionic gonadotropin is a peptide hormone that helps prevent the breakdown of the corpus luteum. This allows the corpus luteum to continue secreting progesterone and estrogen during implantation. As the placenta forms, it eventually takes over the job of secreting hormones for the corpus luteum. The corpus luteum is only preserved when fertilization has occurred; normally, the structure devolves into the corpus albicans. Human chorionic gonadotropin can thus be used to detect pregnancy, as it is only secreted after fertilization.