questionalbel due date

I have questions about the due date calanders... ect. my last "normal" period was aug.  20th 2007 it is usually 4 days long and normal, i was on the ortho evera patch ( for 3 years) i had what i thought at the time was a period on sept 17th that was very light and lasted from early afternoon till the next morning. light enough to not require a tampon only a light pantyliner. i did apply my patch as normal on that thursday the 20th of sept. i wore all three patches as i normally do. i've been ill and swelling in the breasts ( this will be my third i have a 6 and 13 year old.) i did not have a period this month. before i was to apply the patch after my week without i took 3 tests all stating positive. i have an ob appt next week but as im trying to figure things out i do not know if i should be counting last months- 1 day period or forget about that one and use the month before while figuring how pregnate i am? please help clarify how you would determine a due date for my situation. thanks

[alaysha]

I would get an early ultrasound since they are the most accurate for pregnancy dating if you dont know the last day of your period.