I used to use this app called FMC by FertilityFriend.com. I've been using it since 2008, till I found another app that's even easier to use, called MonthlPal.
FMC syncs to FertilityFriend.com, which means if you change your devices, you can sync the data back from the online backup. MonthPal doesn't have that feature yet.
Here are the links if you're interested:
FMC (free) -
https://itunes.apple.com/us/app/free...292760731?mt=8
MonthPal (free) -
https://itunes.apple.com/cn/app/mont...2848?l=en&mt=8