月經週期的基本功能容易總結：

子宮為可能的受精卵植入準備好內膜層。 如果植入沒有發生，子宮內膜破潰並排到陰道。這種破潰和排放稱為經血或簡稱為月經。

當經血已經停止時，子宮內膜開始增生並借此又啟動另一個月經週期。在理論上，描述月經週期能夠從它的任何一個時相開始。例如，為了醫學計算的需要，月經週期被設定為從月經的第一天開始，終止於另一個週期之前的最末一天。但是，從月經週期生理變化的相互關係來看，以卵子生長和子宮內膜逐漸增厚來開始計算它的週期似乎最為適當。

The basic function of the menstrual cycle is easily summarized:

It prepares the lining of the uterus for the possible implantation of a fertilized egg. If no implantation occurs, the lining breaks down and is discharged through the vagina. This discharge is known as menstrual bleeding or simply menstruation.

When the bleeding has stopped, the preparation of the uterine lining and thus the entire cycle starts again. Theoretically, a description of the menstrual cycle could begin with any of its phases. For the purposes of medical calculation, for example, the menstrual cycle is assumed to start with the first day of menstruation and to end with the last day before the next menstruation. However, in the present context it seems most useful to begin with the growth of the egg (ovum) and the gradual thickening of the uterine lining (endometrium).