I just started reading this book, Where Good Ideas Come From: The Natural History of Innovation, one of the central ideas, it seems, is that in order to have good ideas, one should not protect them, but rather connect (i.e. share) them. So, in that spirit, here is my idea:
At the top of the two first mockups you can see two graphs. [A] shows the time between contractions, and how it gradually decreases. The graphs are always updating, since "now" is always shown to the right. The labels in the x-axis mean "time ago", for example "2h" means "two hours ago". Each dot on the line represents a single recorded contraction. In [B], the graph represents how long the contractions are. Typically, this value will increase over time, rather than decrease.
The last mockup [C] shows the "Log mode". Here you can see a simple list of records, and you can edit/delete them (not shown).
So there. Simple app, simple interface. Crappy design, for now (I could certainly use some help here).
As for platform choice, I think the best idea is to build the app using one of the many cross platform frameworks, or even HTML5. The only tricky part is the graph, which would ideally let you pinch-zoom to see a variable time range.
Hopefully a basic version of this will be built very very soon, but I'd love for others to add to my idea, so it becomes a truly great app.