If the doctor says i'm 4 weeks 0 days pregnant from the ultrasound scan, about when did i conceive?

Roughly 4 wks ago: Ovulation generally occurs about 2 weeks after the start of your last menstrual period, which I surmise would have been around 6 weeks ago (approximately). That means you would have conceived about 4 weeks ago (give or take, say, 1 week). The "ultrasound" dates are based on the size of the baby, which in turn are based on YOUR size as well as nutrition, so they're not always exact. Talk to your OB