My wife is pregnant we were having problems before we find out she was pregnant and at the time we were separated, so I have doubts that the child is mine but when she came back after 5 days she told me she was on her periods and when she was done we had sex for several times and after 6 weeks we find out that she was pregnant. The projected due dates from the ultra sound scan says it's the 5th, 7th and 9th of March 2015 so I want to know when did she probbably conceived because the baby might is due in the next coming weeks and I want to buy the baby stuff but I am worried the baby could not be mine so I will be angry if I find out after I have spent my money on somebody else's child please help.

If you have forgiven her and working things out, it does or should not matter.
With that attitude, not buying anything for another man's child, just divorce her and move on, the marriage and relationship is not going to work.

There is no exact dating, even the doctor will be give or take a week. So basically at least a two week window and sometimes more.

Some fetus mature and grow at slightly different rates and more.

If the sexual relationships were close, there is no way to know, tilll after the cihld is born and a DNA test is done.

A real father does not have to be the biological one. He is the one with enough love that the rest does not matter.

There is really no way to know when she conceived. You will have to invest in a paternity test to determine who the biological father is.

With that said, since you are married, in most locations you will be the legal father and it will be required for you to be on the birth certificate.