If your cycles are consistent and around 28 days from period to period, your boyfriend has the better odds. If they are irregular, it could be either guy. Get an early ultrasound (like, next week) and an estimated due date from the ultrasound. You can use an online conception calculator from the e.d.d. If your cycles are irregular, don't let the doctor count forward with a little wheel to tell you your due date, only believe a count that comes from an ultrasound.