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什麼是月經週期？
婦女從青春期開始具有生殖能力，然後到了近50歲時會失去這種能力。然而，即使在她的生育年齡期，只有在每月一次兩側卵巢之一排卵的時候，她才能懷孕。這個按月迴圈的事件和其他有規律的身體變化一起，構成了女性的生殖週期。女性生殖週期最顯而易見的外部徵兆就是月經（按月流血）。因為這個原因，生殖週期也可以稱為月經週期。當然，後一個術語好於前一個術語，因為包括不生育的幾乎所有的婦女（包括不生育婦女）具體體驗得到並且對它很熟悉。

第一次和最後一次月經
女孩的第一次月經（希臘：menarche，初潮）通常在11-13歲之間發生。無論如何，第二次月經可能遠比剛剛來潮的初潮要好得多。在青春期，月經週期相當不規則。只有到了後來，相對準確的月經週期才能夠建立。成年婦女的月經週期維持在28-35天之間。儘管如此，若干次不規則的月經週期總是有可能的和十分正常的。隨著婦女漸漸衰老，不規則的月經週期又會出現。最後，到了絕經期，婦女的月經將完全停止

What is the menstrual cycle?
A woman becomes capable of reproduction during puberty and then loses this capacity in her early fifties. However, even in her fertile years she can conceive only during a certain time once a month when one of her ovaries releases an egg. The monthly recurrence of this event, together with other regular body changes, constitute the female reproductive cycle. Its most obvious external sign is menstruation (monthly bleeding). For this reason, the reproductive cycle may also be called menstrual cycle. Indeed, this latter term has the advantage of referring to a concrete experience which is familiar to practically all women, including those who do not reproduce.

First and last menstruation
A girl's first menstruation (gr.: menarche) usually occurs between the ages of 11 and 13. However, the second menstruation may very well be much farther away than just another month. During adolescence, menstrual cycles are rather irregular. It is only later that some definite pattern is established. In a mature woman, menstrual cycles usually last between 28 and 35 days. Still, some irregularity is always possible and quite normal. The irregularity increases again as the woman grows older. Finally, after menopause, her menstruations cease altogether.