A year later Bridget prepares to be married. At the altar she is greeted by Jack, holding her son, and then moves forward to marry Mark. After the wedding, Mark and Jack, now friends, head to the reception together while Bridget carries her and Mark's son, William.

Who is the daddy in bridget jones baby? Okie... think carefully first, then make a decision:
The relevant information to answer the above question is: After the wedding, Mark and Jack, now friends, head to the reception together while Bridget carries her and Mark's son, William. So, the answer is Mark.