This timeline is an approximation based on a 28- to 30-day menstrual cycle, the average length of a woman’s menstrual cycle. A surge in luteinizing hormone (LH) levels occurs at approximately day 13 of the menstrual cycle, and this stimulates ovulation. The ruptured follicle forms a corpus luteum, which produces large amounts of progesterone that modify the uterus lining so that it is receptive to implantation of an embryo if fertilization has occurred. Fertilization takes place in the fallopian tube. As the fertilized egg travels down toward the uterus, it undergoes a series of early cleavage divisions, producing increasing numbers of progressively smaller cells, known as blastomeres, without changing the overall size of the embryo. The 16-cell–stage embryo is known as a morula, and its cells are indistinguishable. As further cleavage steps occur and the blastocoel develops, the morula becomes a blastocyst. The blastocyst implants into the uterine lining approximately 6–8 days after ovulation. A fully functional placenta does not form until approximately 10 weeks after ovulation. Parturition at full term occurs 40 weeks after ovulation.