Well, it's partly based on the anecdotal research that in the winter DDCs, somewhere between 36 and 41 weeks you always get people complaining that they've got colds, saying they hope they don't go into labour this week because they can't even breathe, etc, etc (Honestly, go see for yourself). SO allegedly a virus passes through 7 people before it mutates enough that you can catch it again (colds and flu are both viral-based) and the incubation period for the cold virus is 2-3 days. Therefore, baby should be here in 21 days or under from the last bad cold of the pregnancy in order to benefit from antibodies to the infections around right now that are transmitted by colostrum.
Except that there seems to be a spate of people getting ill at 33-35 weeks as well, which is just unfair because it throws my results off

But hey, what the heck, it passes the time. Unless you're one of the poor people getting ill twice, of course

Oh, and I'm on day three of a cold, day 12 of niggling pre-labour and all of this would take me straight to my EDD, which is NOT in any way, shape, size or form 42-week related. Which is a good reason for me to cling to this theory like glue. 42 is a bad number...