Hi there

My due date according to my doctor is between 10 and 13 October 2012
When was the baby conceived.

Around Jan 14-25 according to Conception Calculator - estimates probable conception conceive and due dates of baby in pregnancy Baby2see (http://www.baby2see.com/conception_calculator.html#ConceptionCalculator)

OK, but according to the docter I am 18 weeks pregnant, still the same conveivement date?

Dating is all best guess I'm afraid.
Are there any concerns surrounding paternity ?

I agree with Curlyben, its all just a guess (unless you only had sex once ect). According to 18 weeks then the date would have been around January 3 (give or take)