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The menstrual cycle is central to any discussion of fertility and for that reason a brief refresher course can be helpful when beginning an infertility evaluation. The female reproductive cycle is an orderly sequence of events resulting from the interaction of the hormones secreted by the hypothalamus (Gonadotropin Releasing Hormone, GnRH) and anterior pituitary (Follicle Stimulating Hormone, FSH and Luteinizing Hormone, LH) and the sex steroids secreted by the ovaries (estrogen and progesterone, among others) on their target organs. The purpose of the menstrual cycle is (1) to mature and release an oocyte (egg); (2) to provide an appropriate environment for selection and transport of the sperm, fertilization of the oocyte, transportation of the fertilized egg (zygote), and implantation of the five day-old embryo known as a blastocyst; (3) to support the early pregnancy until it can support itself. This dynamic process, when functioning normally, ensures that the proper number of follicles (fluid filled sacs in which the eggs grow) is ready for ovulation and the development of the uterine lining in the presence of pregnancy and/or sloughing of the uterine lining in the absence of pregnancy. The menstrual cycle may be divided into two phases: (1) follicular phase, and (2) luteal phase. The follicular phase reflects the changes occurring in the ovary, whereas the luteal phase reflects the changes occurring in the uterus.

The Menstrual Cycle: Figure courtesy of Professor Alfred Cuschieri
Department of Anatomy, University of Malta

The length of the normal menstrual cycle is defined as the number of days from the onset of one menstrual bleed (typically day 1 of full flow) to the onset of the next cycle. The median length of the cycle in the reproductive-age women is 28 days plus or minus 7 days. Only 10% to 15% of women have menstrual cycles that are exactly 28 days long. The greatest variability in cycle length is observed at the beginning and end of reproductive life: menarche and menopause.

The follicular phase officially begins from the first day of menses and ends at ovulation. A cohort of follicles is recruited during days 1 to 4 of the cycle in response to FSH secretion. Once recruited, the follicles must either ovulate or become atretic i.e. degenerate. One follicle will be chosen (somewhere between days 5 to 7 of the cycle) to ovulate from this cohort. This dominant follicle grows and suppresses the development of the other follicles in this cohort (somewhere between days 8 to 12 of the cycle). The follicular phase of the menstrual cycle ends at ovulation occurring typically on days 13 to 15 of the cycle. Ovulation is the result of the LH surge the onset of which occurs 34 to 36 hours before the egg is released from the follicle.

The luteal phase occurs after ovulation and is characterized by the formation of a corpus luteum (CL) on the residual follicle. The purpose of the corpus luteum is to prepare the endometrium, primed by estrogen levels that rise with the growth of the follicle in the follicular phase, for implantation. The corpus luteum accomplishes this important function by secreting progesterone.
The purpose of the endometrium is to support an ongoing pregnancy. The menstrual endometrium is relatively thin. The secretory-phase endometrium occurs after ovulation as a result of the progesterone secretion of the corpus luteum is significantly thicker and reaches peak levels 7 days after the LH surge, coinciding with the anticipated time of blastocyst implantation.
The corpus luteum degenerates 9 to 11 days after ovulation in the absence of ß-hCG, the hormone of pregnancy. The degeneration of the corpus luteum results in a decrease of estrogen and progesterone levels, and menstruation follows.

Cervical mucus also responds to changes in hormone levels. The purpose of cervical mucus is (1) to protect and nourish the sperm (2) to provide a path for the sperm to enter the uterus and (3) to create a barrier to protect the endometrial cavity during implantation and after the establishment of pregnancy. Immediately after menstruation, cervical mucous is scant and thick. During the late follicular phase under the influence of increasing estrogen levels, the mucous becomes clear, copious, and elastic. As a result of these changes, sperm gain access to the uterus and fallopian tubes, where fertilization occurs. After ovulation, as progesterone levels rise, the cervical mucous once again becomes thick, viscous, and opaque and the quantity decreases, making the entry of sperm or bacteria into the uterus less likely.

References:
Carr B. Disorders of the ovary and female reproductive tract. In: Wilson JD, Foster DW, eds. Williams textbook of endocrinology. 8th ed. Philadelphia: W. B. Saunders Company, 1992:750-751.

Khan-Sabir N, Carr B. The normal menstrual cycle and the control of ovulation. [Internet]. MDtext.com, Inc; c2003 Jun 1 [cited 2004 Jul 27]. Available from:http://www.mdtext.com/female/female3/femaleframe3.htm

McKenzie LJ, Carson SA. Evaluation of infertility, ovulation induction and assisted reproduction [Internet]. MDtext.com, Inc; c2002 Aug 2 [cited 2004 Jul 27]. Available from: http://www.mdtext.com/female/female7/femaleframe7.htm

Speroff L, Glass RH, Kase N. Clinical gynecologic endocrinology and infertility. 5th ed. Baltimore: Williams & Wilkins, 1994:111-119, 183-233.