Early Pregnancy Timeline: Understanding Period and Conception

Question
My girlfriend & I found out she is pregnant last saturday.  She has since verified at planned parenthood.  They say she is like 8wks, but we know she had her period over christmas.  That would mean she was 2wks pregnant and then had her period.  Is that possible, even a chance like 1 in a million?  because if it isn't, it would mean I may not be the father because I only saw her christmas (while she had her period) then not until jan. 23rd and she definately is more than 3wks pregnant.  I want to believe her 1000% but in my research have been told it is absolutely impossible to be pregnant and still have your period.  She insists all women are different and she knows of periods up to 4 months after being pregnant and that no doctor knows if she can be pregnant and still have her period.  I desperately want to believe her and hope 1million percent that she's right and it's surely mine but can't help to wonder.  Can you help?  Please?!


Thank You,

S. M. Salem

Answer
Some women will have implantation bleeding that is mistaken for a period.  These periods are usually lighter than a regular one.
I based her last menstration as 31st which would make her almost seven weeks so I do not think her calculation of 8 weeks is that off.
Then I went to http://www.babycenter.com/calculators/duedate and keyed in her starting date on Christmas and that too makes her around 7 weeks.
So I do not think you should worry about the whole Christmas and her period thing.  I think you are just not understanding the calculation process.
Go go to that link and type in her info see what you get.  You know her  info I don't.
Good luck and let me know.