F-1 students who have completed U.S. degrees in designated science, technology, engineering, or mathematics (STEM) disciplines are eligible for extended periods of OPT. In addition to the standard 12-month OPT period following graduation, STEM graduates may request 17 months of additional OPT, for a total of 29 months.
http://www.murthy.com/2014/04/30/eligible-students-should-request-stem-opt-extension/