So what causes our menstrual cycles to start to change and why? The short answer is that it’s related to how many eggs we have left in our ovaries.
The second post in a three-part series
Understanding how the menstrual cycle changes will make much more sense if you know how a typical cycle works. For a refresher on that, here is the first post in this series.
Every woman is born with all of the follicles (each contains an egg) she will ever have. That number is about 1 million. By the time she gets her first period that number has halved. It takes a couple of years for her cycle to get into a rhythm, but from then on, each month 15-20 follicles develop. Each cycle, one follicle becomes the dominant follicle that releases an egg from the ovary to find its way to the fallopian tube at ovulation in mid-cycle.
Our egg inventory is reduced at a constant rate until about age 37 when it begins to fall off more quickly. At some point, it hits a level that causes the cycle itself to change. There is a constant feedback loop between our ovaries and our brain. Our brain sends an initial message, a hormone called Follicle Stimulating Hormone (FSH) which initiates each cycle. FSH causes the 15-20 follicles to begin to develop and produce estrogen. When the number of developing follicles changes so does the amount of estrogen produced. Did you notice we didn’t use the word drops? That’s because when our cycles begin to change, estrogen doesn’t drop. That is a widely held, but outdated, belief. Read on.
The first thing that happens when we hit a “low inventory” egg level is that our brain senses this lower level and produces more Follicle Stimulating Hormone (FSH) to stimulate more eggs to develop. It’s like a horse slowing down near the end of a race and the jockey (e.g. FSH) cracking the whip, encouraging a little more speed/effort. This often causes the levels of estrogen produced early in the menopause transition to be higher than they’ve previously been and this produces symptoms in some women. In addition, this “develop more eggs” message often causes more than one egg to be released and results in more twin pregnancies as we age.
At the same time, studies show that progesterone levels start to decrease. As we age we start to ovulate less frequently. It is debated whether you can have normal cycles but not ovulate. Most scientists believe that not ovulating produces a noticeably irregular, longer cycle with atypical bleeding at the end. Others disagree. They believe that a cycle of normal length with a typical amount of flow during your period can be anovulatory i.e, without ovulating.
“Bottom of the barrel” eggs are thought to be low quality. So it becomes harder to conceive in the late reproductive years. If either ovulation does not occur or a lower quality egg is released, little to no progesterone is produced and the second wave of estrogen is not as robust either.
We’ve spent some time trying to illustrate these initial changes associated with higher estrogen and sometimes lower progesterone on the left side of this infographic. We hope as you take a look you might see familiar symptoms that we talk more about in part 3.
