D.N.
I kept a notebook for my triplets. It worked well. Whenever we went to the doctor, I pulled it out of the diaper bag--kept there all the time--and recorded weight, height, percentages, shots etc. I also had a section for emergency things that came up and was able to mark down any firsts so it could be saved for later. The great thing was I was also able to write down what kinds of foods they like and any recommends from the doctor so I could refer to it later. Going to the doc with multiples can be crazy so being able to jot down quick notes is a big time saver.
I also have a notebook I keep for my 17 month old and record the same info.