If a doctor looks at the baby by ultrasound and, due to its measurements, says "You are 6 weeks pregnant," he or she is counting from the first day of your last period, or the calculated first day of an average woman's last period who has an embryo that size in her uterus. That would mean, as RockRose says, if the doc said you're 6 weeks along, you are at that time 4 weeks from conception.