The Menstrual Cycle Introduction

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A. Introduction

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5. The Menstrual Cycle

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The development of a mature oocyte requires the endocrine interactions of the menstrual cycle. The

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length of the menstrual cycle (Latin = mensis = ‘month’) is the number of days between the first day of

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menstrual bleeding of one cycle to the onset of menses of the next cycle and has an average duration of

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~29.5 days or approximately one month (Figure 13). However

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on an individual basis

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of the menstrual cycle varies with only 10%-15% of all cycling women having an 33 154ay cycle.

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Most women who menstruate regularly have cycles that last from 25-30 days in length

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with older wormen

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having shorter cycles than younger women (Fehring et al.

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2006). The key to fertility is not having a cycle

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length of exactly 29.5 days but consistently having cycles of the same duration. Erratic menstrual cycle

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lengths in the same woman are associated with infertility.

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The menstrual cycle is divided into the following two major phases:

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a. The follicular phase lasts approximately 14 days

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during which time there is a daily increase in

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follicular growth and estrogen production. The length of the follicular phase appears to depend on the

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rate at which the principal preantral follicle matures

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since it is the main source of estrogen and

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therefore a major regulator of cycle length. Menstruation occurs at the beginning of the follicular

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phase and the day on which menses first appears is considered Day 1 of the menstrual cycle. On the

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14 day of a 28-day menstrual cycle ovulation occurs and the follicle is transformed into the corpus

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luteum. Ovulation marks the end of the follicular phase and the beginning of the luteal phase. The

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onset of the LH surge precedes ovulation by 36 hours and LH levels peak 10 to 12 hours prior to

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ovulation (Fritz and Speroff

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1982).

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b. The luteal phase is associated with increasing production of progesterone and some estrogen that

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prepares the uterus for implantation. The luteal phase is relatively constant in all women and will end

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in approximately 14 days with the appearance of menses

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if implantation does not occur. The cycle

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will repeat with the resumption of a new follicular phase (Figure 13).