The facts on menstrual cycle phases

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By Dr Melisa Holmes, OB-GYN & Founder of Girlology

The menstrual cycle is a mystery to a lot of people, but it needn’t be! Half of the earth’s population has a menstrual cycle and without it, the human species wouldn’t even exist! Obviously, it’s a common, normal, and important part of life, so shouldn’t we all understand it? We think so. Let’s break it down.

The purpose of the menstrual cycle is to prepare the uterus for pregnancy. Under the influence of hormones, mostly oestrogen and progesterone, the menstrual cycle is a series of changes that the uterus and other female reproductive organs go through about once a month.

If a pregnancy occurs, the menstrual cycle stops cycling. When there’s no pregnancy (which is most of the time), the menstrual cycle repeats itself over and over again. The menstrual cycle repeats from puberty (starting anywhere from the age of nine to 16) to menopause (which is when periods naturally stop – usually somewhere around the age of 50).

The four phases of the menstrual cycle

There’s a lot of science and anatomy involved in the menstrual cycle (bodies are pretty cool) and explaining the menstrual cycle can be complicated, but it’s easier if we break down it into four phases:

Follicular phase

On the first day of a period, a ‘new’ cycle begins as a hormone called follicle stimulating hormone (FSH) starts to rise. FSH tells the ovaries to get some eggs ready to release. This is called the follicular phase because the eggs grow inside follicles (which are small fluid-filled cysts), and the follicles release the hormone oestrogen during this phase. Oestrogen causes the endometrium to get thicker and fluffier so it will be nice and cosy for a fertilised egg if one comes its way.

The follicular phase lasts from the first day of a period to ovulation, which takes anywhere from one to three weeks in normal cycles. It’s the most unpredictable and variable phase of the menstrual cycle.


When an egg is released from the ovary, we call this ovulation. The changing oestrogen levels from the follicular phase cause the level of another hormone called luteinising hormone (LH) to rise quickly. The surge in LH triggers one of the follicles (usually the largest) to pop open and release its egg, which heads into the fallopian tube and starts its journey towards the uterus. It is your body’s way of prepping for a possible pregnancy – if the egg is fertilised, this means you may become pregnant. If the egg is not fertilised, it will dissolve within about two days.

Most women can’t feel exactly when they ovulate, but some may notice mild cramping for a day or two (this is called ‘mittelschmerz’). It’s also normal for vaginal discharge to look clearer and stretchier for a few days around the time of ovulation.

Luteal phase

After ovulation, the follicle the egg grew in (called a corpus luteum) continues to produce another hormone called progesterone. Progesterone keeps the endometrium healthy and is required to support the growth of a pregnancy. But if there is no pregnancy, the corpus luteum stops producing progesterone, and the falling progesterone levels cause the entire lining of the uterus to shed so it can start over again with a new cycle, preparing for another egg.

The coolest thing about the luteal phase is that it is almost always the same length of 14-15 days in all cycles and for all women with periods. That means if you could predict your next period precisely and then count back two weeks, you would know when you’re ovulating!


Menstruation or the menstrual period is the period of time during which the endometrial lining is released (that’s why we call it a period). Periods normally last anywhere from three to seven days as the endometrium, blood and fluids are slowly released. Once the period begins, guess what? The cycle of hormones starts all over again with the follicular phase, preparing another egg and the endometrium.

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Some eggs mature. One egg ovulates. The egg travels through the fallopian tube into the uterus. If there’s not a pregnancy, the uterus sheds its lining as a period, and the cycle starts all over again. There you have it – that's the menstrual cycle. Sometimes an illustration can help!