Drs will calculate your due date by the date of your last period, not by ovulation. So by the time you ovulate you will be about 2 weeks pregnant, and by the time you miss your period, you will be about 4 weeks pregnant. That would make you 5 weeks and 1 day pregnant right now and according to the due date calculator, your estimated due date should be october 20th
Of course your OB or midwive , whichever you are using, will most likely adjust it once you have an ultrasound by measuring the size of the baby, by doing that they can get a better estimate of just how old the baby is. Of course due dates are always an estimate. They may end up adjusting you multiple times (though most likely not) and you baby will probably measure big somedays and smaller other days. Ultrasounds are usually the most accurate way of telling you how big the baby is but they can and do vary. At my last scan with my first (35 weeks) the ultrasound tech told me my baby was already 5 lbs and that if he continued to grow at a regular pace I should have expected an 8.5 lb baby but he came out at 6 lbs 13 ounces. oh and he was a week over due!
hopefully this helps you..probably more info than you needed =P