If my wife due date is 28.5.2016 when did she get pregnancy

Grab a calendar: The official start of a pregnancy is 40 weeks before the estimated due date, plus or minus a week. The conception day is 38 weeks before the estimated due date, plus or minus a week. May 28, 2016 is Saturday, so just count backwards on the calendar... and the conception estimate will simply be between 37 and 39 Saturdays ago. Also, it should not matter when she got pregnant... congratulations!