Ok so I went for an early ultrasound yesterday and the baby measured 5weeks 5days.. so based on that who would more likely be the father ??a or b? The conception date would've been dec. 15th
Until you get an early ultrasound that tells you an estimated due date, you have to assume that either guy could be the dad.
Some women don't ovulate on the schedule provided by an ovulation calendar and pregnancy calculator. (All an ovulation calculator tells is when an average woman might have ovulated.)
Best thing to do is get an ultrasound around January 15-20, and tell the doctor you don't know when your last period began, and ask for an estimated due date based only on the embryo's measurements and developmental markers. Then take that due date home and plug it into a conception calculator online or just count back 266 days, and that will give you an estimated conception date. I just hope the date it gives isn't the 13th or 14th, since both guys' sperm could have been alive in your system at the same time on those days.
If the ultrasound does point to a day when both guys could be the dad, there are prenatal DNA testing companies out there that (for a big fee) can do a blood draw from the mom's arm and swabs for the possible dad(s), and confirm paternity before the baby is born. If you do go that route, Ravgen and the DDC are reliable labs. Most women prefer to save the money and test after the baby comes for only a tenth or so of the cost.