| On the first day, the instructor tried proving the "if you have 23 or more people in the room, odds favor that two of the 23 will have been born on the same month and day.*
As there were 27 of us in the room, we happened to prove him right this time.
Similarly, I'm not entirely surprised that an event with (nearly) regular timing should happen in common to several members of a large group.
Considering this particular event is subject to stress patterns, illness, etc. it would seem that the premise that women who are in close proximity over a large portion of time and sharing similar life tempos would become more regular with respect to one another.**
*For y'all who're picky about such things, this premise assumes an even birth rate distribution across all days of the year.
**The preceding was an opinion from someone carrying XY chromosomes and wearing a wedding ring, and I make no claim to be expert in the latter either. :) | 
