Many women in the early stages of labor do not keep track of their contractions, simply because having a contraction and operating a stopwatch, together with doing the simple math does not go down well with contraction pains.
I propose (and will soon build) a device that fits on a keychain and use a single button that needs to be pressed during contractions. The user will have two forms of display, namely an OLED display to show rolling average of contraction duration and spacing, together with 3 LED's to display other on-the-fly information or to attract attention.
The device will use a small LiPo battery, and although it can be made smaller using custom PCB's, it will rather be made out of directly available electronics, using the Trinket Pro as the brains.
Put it as you may, I prefer my hardware to be DIY. I'm pretty sure the only "off the shelf" part I have in my toolbox is one lonely Arduino Uno. Okay, I'm not talking about winding my own resistors or building speakers, but I am talking about using an ATmega328 with some circuitry instead of an Uno, using a MAX1555 with everything needed vs buying a "battery back pack".