How do I calculate my due date in my case?

I am pregnant and my last period was on July 17th and though I have regular periods monthly it just jumps dates since I've lot 10 kgs(22 pounds) in the past 6 months,for eg.my periods in Jan 2009 was on 5th,Feb on 15th,March on 17th,May on 25th,June on 19th and July on 1th..so in this should I take the date from June to July as the lenght of my cycle or what?I am so confused.