You're 13 weeks 4 days pregnant? This was given to you by a doctor looking at the baby with an ultrasound? And on November 25 you had a negative urine test, and now you are trying to rule out sex on October 24 as producing the pregnancy? I would certainly rule it out if the doc did an ultrasound scan and gave you your weeks count, since used an ultrasound, since if you had gotten pregnant from sex on October 24, they would be telling you that you are in your 16th week.