[Ben77]

We had sex between August 27th - 31st 2013, Girlfriend says Sonar scan reveals due date as 15th May, is this possible?

[smoothy]

You mean Sonagram, Sonar is what ships and submarines use.

The doctors guess will be accurate to plus or minus 2 weeks meaning effectively the entire month of May not just that specific day. That is nine months so yes it is possible.

[ScottGem]

Certainly its possible. Doctors generally calculate a due date as 280 days from the onset of their last period, which is 40 weeks. But that estimate can be off by as much as 5 weeks, depending on whether the woman is sure when they started their last period and other factors. Calculating a due date is an inexact science as is calculating a conception date. A May 15th due date means that conception occurred sometime within a period of 8/15 -9/15. So yes, its possible.

If you are concerned whether the child is yours or not, the only way to confirm it is with a DNA test after the child is born.

[J_9]

The doctor calculates pregnancy according to her last monthly period (LMP) not by when you had sex. So, yes, it is entirely possible.