For what it is worth, here is what the conception
calculator says (assuming when you say your last menstrual period 'was' May 5, you mean it began May 5):
First day of last menstrual period: May 5, 2012
Probable date of ovulation: May 19, 2012
Possible dates of conception: May 15 to May 23, 2012
Due date: February 9, 2013 (40 weeks)
This is just counting by the period alone, and the "probable date of ovulation" is just counting out 14 days from the first day of the peri