You can use a special question type called a hidden value to store values from other forms and carry out calculations without it being visible to the mobile worker. The important field is the calculate field, which you can put calculation expressions. Some common examples include:
The hidden value will hold the null value '' until its display condition is met. Once its display condition is met, it holds the calculated or loaded value.
For further guidance on doing calculations please see Hidden Values Tutorial Part 1: Adding Two Questions Using a Hidden Value and Hidden Values Tutorial Part 2: Calculations Using a Date.
Other calculations can be found on this page.
*EDD calculations: http://www.perinatology.com/calculators/Due-Date.htm and http://nrhm-mctsrpt.nic.in/MHFW/UI/FAQAnc.aspx