Help on when conception occured

Patient: Hi im in my 12th week of pregnancy and im trying to figure out my concepection date. I had sex on the 1st of oct. Im wondering if it was that sex or a later sex. My due date is july 6, 2015 confirmed by ultrasound at 8weeks. Thanks for your help this is very important for me to know.

Doctor: Hello,The most important thing for calculating dates is your last menstrual period which you haven’t mentioned. Ultra sonological inferences and dating often has a discrepancy of 10-15%.Also important is your length of cycle and if its regular or not.Any ways considering the ultrasonological age , your lmp by naegle’s formula should be 29 september 2014.Considering this you were in your safe period on 1 oct, and definitely cannot conceive the. Hence a sex later during or near ovulation in that cycle has caused this pregnancy.i hope i have answered your query in detail,wishing you safe pregnancy,regards