The Wheel is a gestational age calculator (or pregnancy wheel) like the ones doctors and pregnant women use everyday. It determines your due date based on the first day of your last menstrual period. It also simplifies the precise calculation of the gestational age of a fetus at any stage of the pregnancy. It costs $15 to download and can be found if you search "The Wheel" in iTunes.
In general iPhone apps are compatible with iPod touch and the iPhone.