A fertilized ovum divides by mitosis , and in about a week produces the blastocyst that implants into the endometrium. If the blastocyst successfully implants in the uterine lining, then a pregnancy occurs. At the start of pregnancy, the corpus luteum continues secreting progesterone under the influence of human chorionic gonadotropin (HCG) secreted by the implanted embryo. The continuous secretion of progesterone maintains the endometrial lining. As the pregnancy continues, the placenta gradually takes over the reproductive endocrine functions that were performed in the nonpregnant woman by the ovaries. In addition to its endocrine function, the placenta is vitally important in providing nourishment and waste removal for the growing embryo/fetus throughout gestation.