system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do women living together end up with synchronized menstrual cycles?

My 16 sister year old sister doesn't know, and I'm curious.
gpt
They don't.

A woman's biological cycle is not clockwork.  The whole "28 days" thing is an average.  Some women have 26 day cycles.  Some 30.  Some women float between 25 and 32.  Some are even more random.  And even women who start their period every 28 days like clockwork rarely start it at the same time on that day - noon one time, 8am the next time, 8pm the next time.. and so on.

If you have two women, one with a 29 day cycle and one with a 26 day cycle, then at any given point in time their cycles will appear to be getting closer together.

It's.. kinda like the turn signals on a car.  Ever watched two turn signals not anywhere close to each other slowly start to sync up, then they appear to be blinking at exactly the same time?  But what happens after that?  They start driving apart again, because they aren't actually syncing up with each other, they just happen to be in a part of the cycle where they appear together.

Same thing with women, only it takes a month to get a data point.  Combine the length of time needed to gather data with confirmation bias, and you'll have women swearing their cycles are getting closer when they're actually getting farther apart.   Because they're starting to sync up with the preceding one.