Menstrual cycle

The menstrual cycle refers to the regular changes in the activity of the ovaries and the endometrium that make reproduction possible.

The endometrium is the layer of tissue lining the inside of the uterus.

This lining consists of a functional layer, which is subject to hormonal changes and is shed during menstruation, and a thin basal layer which feeds the overlying functional layer.

The menstrual cycle actually consists of two interconnected and synchronized processes: the ovarian cycle, which centers on the development of the ovarian follicles and ovulation, and the uterine or endometrial cycle, which centers on the way in which the functional endometrium thickens and sheds in response to ovarian activity.

Menarche, which refers to the onset of the first menstrual period, usually occurs during early adolescence as part of puberty.

Following menarche, the menstrual cycle recurs on a monthly basis, pausing only during pregnancy, until a person reaches menopause, when her ovarian function declines and she stops having menstrual periods.

The monthly menstrual cycle can vary in duration from 20 to 35 days, with an average of 28 days.

Each menstrual cycle begins on the first day of menstruation, and this is referred to as day one of the cycle.

Ovulation, or the release of the oocyte from the ovary, usually occurs 14 days before the first day of menstruation (i.e., 14 days before the next cycle begins).

So, for an average 28-day menstrual cycle, this means that there are usually 14 days leading up to ovulation (i.e., the preovulatory phase) and 14 days following ovulation (i.e., the postovulatory phase).

During these two phases, the ovaries and the endometrium each undergo their own set of changes, which are separate but related.

As a result, each phase of the menstrual cycle has two different names to describe these two different parallel processes.

For the ovary, the two weeks leading up to ovulation is called the ovarian follicular phase, and this corresponds to the menstrual and proliferative phases of the endometrium.