Fertility - The Menstral Cycle - Bay Area Northern California Infertility Fertility Specialists,  Egg Donor, OB GYN, IVF, IUI, Embryology, Infertility Testing, Male Reproductive Health, ICSI, PGD
 
The female reproductive system is a  wonderfully complex system involving continuous communication between the brain  centers and the ovary. Hormones secreted by the hypothalamus, the pituitary  gland, and the ovary are the messengers which regulate the monthly cycle.
The Hypothalamus and the Pituitary Gland
The hypothalamus is located  centrally in the brain and communicates by way of an exchange of blood with the  pituitary gland. Several neuroendocrine agents, or hormones, are produced by  the hypothalamus. The most important hormone for reproduction is called  gonadotropin releasing hormone or better known as GnRH. It is released in a rhythmic fashion every 60-120 minutes.
GnRH stimulates (causes) the  pituitary gland to produce follicle stimulating hormone (FSH), the hormone responsible for starting follicle (egg)  development and causing the level of estrogen (the primary female hormone) to  rise. Luteinizing hormone (LH), the  other reproductive pituitary hormone, aids in egg maturation and provides the  hormonal trigger to cause ovulation and the release of eggs from the ovary.
The Ovary
The main function of the ovaries is  the production of ooctyes (eggs) and hormones. At birth, the ovaries contain  several million immature eggs. No new eggs will be developed. These eggs are  constantly undergoing a process of development and loss. Most will die without  reaching maturity. This process of egg loss occurs at all times, including  before birth, before puberty and while on birth control pills. The ovary  undergoes a constant process of oocyte depletion (loss) throughout its  lifetime.
As the levels of FSH and LH in the  blood increase with puberty, the eggs begin to mature and a collection of fluid  (the follicle) begins to develop around each. The first day of menses is  identified as cycle day one. Estrogen is at a low point.  Therefore, the pituitary secretes FSH and LH, a process which actually begins  before the onset of menses. These hormones, in turn, stimulate the growth of  several ovarian follicles (each containing one egg). The number of follicles in  the "cohort" of developing follicles each month is unique to each  individual. One follicle will soon begin to grow faster than others ―the dominant follicle.
As the follicle grows, blood levels  of estrogen rise significantly by day seven of the cycle. The increase in  estrogen begins to inhibit the secretion of FSH. The fall in FSH allows the dying  away of smaller follicles. In essence, they become "starved" for FSH.
Ovulation
When the level of estrogen is  sufficiently high, it produces a sudden release of LH, usually around day  thirteen of the cycle. This LH surge (peak)  triggers a complex set of events within the follicles that result in the final  maturation of the egg and follicular collapse with egg extrusion. Ovulation,  takes place 28-36 hours after the onset of the LH surge and 10-12 hours after  LH reaches its peak. The cells in the ovarian follicle that are left behind  after ovulation undergo a transformation and become the so called corpus luteum. In addition to estrogen,  they now produce high amounts of progesterone to prepare the lining of the  uterus for implantation.
The Luteal Phase
The luteal phase, or second half of the menstrual cycle, begins with  ovulation and lasts approximately fourteen days (typically 12-15 days). During  this period, changes occur which will support the fertilized egg (embryo)  should pregnancy result. The hormone responsible for these changes is progesterone  and it is manufactured by the corpus luteum. Under the influence of  progesterone, the uterus (womb) creates a highly vascularized bed for a  fertilized egg. If a pregnancy occurs, the corpus luteum produces progesterone  until about ten weeks gestation. Otherwise, if no embryo implants, the  circulating levels of hormone decline with the degeneration of the corpus  luteum and shedding of the lining of the uterus (endometrium) leading to bleeding  (menses).
The Uterus
The lining (endometrium) of the uterus  prepares each month for the implantation of an embryo. This preparation occurs  under the influence of estrogen and progesterone from the ovary. If no  pregnancy develops, the endometrium is shed as a menstrual period, about  fourteen days after ovulation.