There are two cycles that needs to be discussed. The Menstrual Cycle/Uterine Cycle and the Ovarian Cycle. Both are controlled and regulated by the hormones your body makes and secretes. First of all, not everyone has the same menstrual or ovarian cycle. Some are shorter and some are longer. Same thing for the different phases within the two cycles.There are three different phases in the Menstrual cycle: Menstrual flow phase, Proliferation phase, and the Secretory phase. The average length for the menstrual cycle is 28-32 days, but in some women it can be shorter and in some longer. Then Menstrual Cycle is counted from the first day of menstruation to the first day of the next menstruation.
Menstrual Cycle/Uterine Cycle. The first day of menstruation (or bleeding) is the first day of your cycle. This is the Menstrual flow phase. The bleeding is from the cause of the shedding of the endometrium, which is the lining of the uterus and its blood supply. During your cycle, the endometrium layer thickens for the possible implantation of the embryo, the fertilized egg by the sperm. When there is no fertilization, the body realizes that the thickened endometrial layer is not needed, so the body sheds it. Therefore, our bleeding. This phase can last for a few days to a week.
The next phase is the Proliferation phase. In this phase the thin layer of the endometrium starts to thicken again and become rich with blood supply. The thickening is the body’s way of getting ready for possible fertilization of the egg and the sperm. This phase can last from a week to two weeks.
The last phase is the Secretory phase. In this phase the endometrial layer continues to thicken and the arteries enlarge with blood supply. When there is no fertilization, the endometrial layer breaks down and is shed from our body, which then starts the whole cycle again. The secretory phase can last up to two weeks.
Ovarian Cycle. The Ovarian Cycle is the the growth and maturation of your follicles in your ovary, or your eggs. This cycle also has three phases. The first phase is the Follicular phase, in which your egg grows along with layers of tissue around it as it travels through the ovary. The length of the follicular phase varies but it starts when your menstrual cycle starts. When the egg reaches maturity, it is released from the ovary, starting the Ovulatory phase. During the Ovulatory phase the egg is released into the Fallopian tube, ready for fertilization. The egg lives up to a day after it is released from the ovary. The last phase is the Luteal phase. When the egg is released in the ovulatory phase, the layers of tissue that it leaves behind in the ovary is changed into the corpus luteum. The corpus luteum secretes hormones but as it dies and disintegrates the levels of hormones also drop. The luteal phase can last from 13-15 days. The length of the ovarian cycle coincides with the woman’s menstrual cycle and the Luteal phase parallels the Secretory phase.
Hormones. The two cycles: Menstrual and Ovarian are both controlled and regulated by your hormones: Gonadotropin-releasing hormone (GnRH), Follicle-stimulating hormone (FSH), Luteinizing hormone (LH), Estrogen, and Progesterone.
In the follicular phase of the ovarian cycle, the hypothalamus secretes GnRH, which causes the anterior pituitary gland to secrete small amounts of FSH and LH. The FSH hormone stimulates follicle growth (egg growth) in the ovary. These growing eggs then secrete Estrogen. As the eggs and follicular cells grow and mature, high levels of Estrogen is released, which stimulates the hypothalamus to release more of GnRH, which then causes more release of FSH and LH. This increased surge in LH causes the final maturation of the egg and follicles and ovulation occurs, where the egg is released. After ovulation, the LH changes the follicular cells that were left in the ovary into the corpus luteum, causing it to mature and grow. The Corpus Luteum then secretes Estrogen and Progesterone. As the corpus luteum matures, higher levels of Estrogen and Progesterone are released, causing the continued thickening of the Endometrium. This higher levels of Estrogen and Progesterone then prevents the hypothalamus and the pituitary gland from releasing more of FSH and LH. When the LH levels drop, the corpus luteum starts to disintegrate since it had needed the LH to keep on maturing. The disintegration of the corpus luteum also causes the levels of Estrogen and Progesterone to fall, causing the lining of the Endometrium to be shed. The dropping levels of Estrogen and Progesterone frees the hypothalamus and pituitary gland and so the hypothalamus secretes GnRH and starts the cycle all over again.