Menstruation
Menstruation has no connection to the moon, and women don’t sync up due to cohabitation or detecting each other’s hormones.
These are just events that roughly follow a monthly cycle. Over time they might appear to converge, but they will then diverge without people thinking about it.
Our brains are always looking for patterns, and this is a false positive like with pareidolia.
Story credit – Reddit/[deleted]