Chicago Birth Certificate Film Oddities

When I first started doing lookups, I had to look at up to four films to find some of the death certificates in the teens--until I figured out the pattern and the exceptions to the pattern. Now I need to do the same thing with the birth certificate films. Apparently, for some years (in the teens?), the December birth certificates for one year were filed with the certificates from the beginning of the following year. What this seems to do is push the months off the expected pattern. For example, with three repeats of certificate numbers, I'd expect them to be Jan-Apr, May-Aug, Sep-Dec, but it looks like sometimes April may actually be in the second group: Dec-Mar, Apr-Jul, Aug-Nov? Time for a bit of new sleuthing.