Need Help Baby dates?

Hi there i am 16 weeks 6 days pregnant today ( tuesday 16th march 2010) ... im very confused as to when i fell pregnant as me and my long term boyfriend broke up on the 27th novemeber and i went with another man and had with him on the 3rd December... we are just confused as to who's baby it is ... i have asked my friends and midwife all the same question and so far theres a mixture of answers and i just wanted to know if anyone could help me out ... many thanks roxy