Have you had an ultrasound? They are most accurate, and the early ones are best. Babies grow at different rates so they may measure bigger or smaller on the later ultrasounds. Also ask you ob what your official due date it. They usually don't change due dates very often.
Is there more than one candidate for dad if there is a difference between February 5, 7, 9 or 12? Because even the best conception calculators usually give you a range of about a week. A lot will depend on how regular your cycles are, and how reliably you think you ovulate at a certain point in the cycle. Women can ovulate as early as day 7 or 8 of their cycle, and if they have a long cycle could ovulate as late as day 15.
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 period.
Well the last day of my period was may 5th. It started may 3rd. My periods weren't regular and most of the time only lasted 4 days. I always stared them at the end of the month nd carried over to the next month.
Due dates are all just estimates being that they are all so close together its really not a big deal, and sometimes they will change after ultrasounds. Besides the babies come when they are good and ready reguardless of their due date.
If you were someone who had 28-day cycles and ovulated all the time on the 14th day (first day of your period counting as day 1), then the following schedule would be right. But since you don't have cycles that are so regular or any proof of when you ovulated, you really have to ignore the "probable date of ovulation" and just go with the third line, the "possible dates of conception." Best you can know is a range, not a day.
First day of last menstrual period: May 3, 2012
Probable date of ovulation: May 17, 2012
Possible dates of conception: May 13 to May 21, 2012
Due date: February 7, 2013 (40 weeks)