system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] When women are ovulating, how does their body decide which egg goes next? Is there a specific order or do they just randomly fall out?
gpt
There is an element of randomness.

The group of cells that produce a single "egg" is called a follicle.  They respond to a hormone known as Follicle Stimulating Hormone (or FSH for short), released by a part of the brain.  When more FSH is released in the early part of the menstrual cycle several follicles, around 15 to 20, will start to respond by becoming more mature.

As the follicles mature they start to produce other hormones of their own.  These hormones do two things (well, more than two, but two that matter for this topic) at the same time - they start to reduce the amount of FSH that is released, and they start to make that particular follicle more sensitive to whatever FSH does still get released.

It might then be easiest to think of this as a competitive process.  The follicle that has the quickest and strongest response both matures faster and prevents the other follicles from responding to the signal.  Eventually this dominant follicle matures to the point of ovulation which results in another set of hormone changes that allow the rest of the cycle to proceed.  Some of the follicles that matured to a certain point but didn't become the dominant follicle will "die" at this point, and a new set will start to mature in the next cycle.