Biology of a Cycle, the short-short version

There are major hormone changes happening in perimenopause. Let me back up. There are major hormone changes happening constantly in a woman’s body from puberty through menopause. Our bodies get in to a nice rhythm once periods are regular, sometime in our teens to our early 30’s on average. For most women, periods are a monthly hiccup where the daily routine changes a bit for a few days but we march through and go about our business. Please know that I’m talking about the majority of young women who have normal cycles. I see women every day in clinic that struggle with their cycles; they are a huge upheaval in their lives and a major challenge each month. I’m not talking about these women right now. Most young women find their cycles to be an annoyance and nothing more (and that, perhaps, is a topic for another discussion!).

In a normal cycle, you have two distinct phases. The whole cycle lasts on average about 28 days, with each phase lasting 14 days. There is huge variability here. A normal cycle can be anywhere from 21 to 35 days long, which would make the 2 phases more variable in length. Almost always, the second phase is 12-14 days, which means the first phase is usually the one that is longer or shorter. Anyway, the take-home point is there are 2 phases of a menstrual cycle.

The first phase is called the Follicular phase. Day one of your cycle is the first day you have a normal bleed (spotting doesn’t count!) and the first day of this phase. At the same time you have your period, your ovaries begin again to get ready to ovulate an egg. An egg follicle starts to mature and is typically ovulated or hatched around the 14th day of a normal cycle. Again there’s variability here. A healthy ovulation occurs between days 11 -17 of a cycle and usually depends on how long the full cycle is. Many hormones are involved in the menstrual cycle but the main player here is Estrogen.

Once an egg is ovulated, the 2nd phase begins. It’s called the Luteal phase. The shell of the egg, called the corpus luteum, sticks around for a while and makes a hormone called Progesterone. It’s the main player of the luteal cycle. If there is no pregnancy, the luteum gets smaller and smaller, the body makes less Progesterone and it signals another period to happen. The whole cycle then begins again.

I’ll come back to this biology lesson frequently in future posts. These two phases and the changes that occur with them as we age are key in understanding perimenopause and its symptoms.

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