The Cycle Is a Wave, Not a Number

Some of you may have taken your temperature the moment you woke up. One day it reads 36.4°C; a few days later, 36.7°C.

Two numbers alone say nothing. Only when you line up several days in a row does a line appear. At some point it climbs a step, stays there for a few days, and then comes back down.

The same 36.7°C is an ordinary number on one day, and a number that tells you something on another. What separates the two is not the number itself but where that number sits.

A Single Number Has No Direction

36.7°C by itself is neither high nor low. Only when a few days are attached before and after does it become visible whether it is rising or falling.

That is why, in a woman's body, a question comes before “does this fall within the normal range.” “Where in the cycle was this measured?” Leave this question out, and two people can look at the same number and tell entirely different stories.

This is also why our intake form asks when period pain occurs — before the period, on the first day, during, or after. The same period pain lands differently depending on where it falls in the cycle.

It Rises and Falls in Order

The number does not rise and fall at random. There is a sequence.

From the end of the period to ovulation, estrogen is dominant. During this stretch, the body works mainly toward building and storing. The lining of the uterus (endometrium) thickens, and collagen increases in the skin.

After ovulation, a corpus luteum forms where the egg was released, and the progesterone it produces becomes dominant. Now the body shifts from building toward maintaining and releasing. This is also when body temperature climbs a step. Progesterone acts on the region of the brain that governs temperature (the hypothalamus) to raise the baseline slightly, and that state continues until the next period.

The two hormones are not a switch, one turning on as the other turns off. Both are present throughout; what shifts is which one carries more weight. And these two are not the only ones involved — they also push and pull against thyroid regulation.

This Is Not the Uterus's Work Alone

The places that change along the cycle are not limited to inside the uterus.

There is data measuring 85 test items across the cycle in 893 healthy women. Beyond the sex hormones, differences turned up in many items. Sodium, chloride, and parathyroid hormone were higher around the start of the period, while insulin, total cholesterol, and white blood cell count were higher after ovulation. The differences were not large, however, and the researchers themselves concluded they were not large enough to meaningfully affect how test results are read.

What stands out is not the size of the numbers but the direction. Even items that seem to have nothing to do with the uterus moved, a little at a time, together with the cycle.

The skin is the same way. Measuring how much moisture leaves the skin (transepidermal water loss) shows a difference between around ovulation and afterward.

So this is hard to file away as “a matter for period days only” or “a matter for gynecology only.”

This Is Not the Same as Being Irregular

“They say a woman's body is just like that.”

Hearing this for a long time makes you regard your own body as something hard to trust. What happens every month starts to feel like a series of separate accidents, and then it becomes hard to see where to even begin.

But a number rising and falling does not mean it is out of control. A body that climbs and descends in a set sequence is a body that is being regulated.

So the question here is not how far it rose or fell. It is whether the sequence is being kept. When the order is disturbed, when a step that should be there is missing, when something that used to come stops coming — that is where the story changes.

A woman's body is not an unstable body. It is a body regulated along a cycle.

When Testing Comes First

A period that has not come for more than three months, a sudden and marked increase in flow, bleeding at a time unrelated to the cycle, and the possibility of pregnancy. These four should be checked at a gynecologist's office before asking where in the cycle you are.

If you are taking birth control pills or other hormone-related medication, that medication itself changes the wave of the cycle. The sequence described here is drawn for a body not on such medication, so it is hard to map onto it directly.

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This month being different from last month does not mean the body acted on its own.

Line up which point of the cycle each month passed through, and two months that looked different often turn out to be different positions on the same curve.

Once you know that position, things that seemed to happen separately begin to connect into one line. The story of how the baseline itself shifts at menopause is the story of this very curve changing.

Written by Dr. Heo Ji-young (PhD in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)