MizBrown, It all depends when the ultrasounds were done. The earliest one around 6 weeks is the most accurate. from there on they lose accuracy and after week 18 they are totally useless to determine conception, only to monitor the baby's growth and predict a birth date.
Unfortunately with Aug 25/26 as the due dates, either partner could be the father. Partner B is however more likely to be the father.
Best of luck!