How do i work out due date my period was last weeks in July?

http://www.americanpregnancy.org/duringpregnancy/pregcalc.html

use the above pregnancy calculator, there are several other ones out there as well, this one will give you a due date and estimated day of conception as well! Just estimate the FIRST day of your LAST period and put that in the first box (mine was 07/06/2009) and then estimate how many days apart your periods are (mine are roughly 30-32) most are 28 day cycles. The Doctor gave me a due date of Apr 12, 2010, but this is close by telling me Apr 15, 2010. All of the calculators are close but different, but it is only a number because actually only 5% of women give birth on their due dates!!!