1)  If you have a 28 days cycle--

Add 9 months & 7 days to the date of your last period.
This is your expected date of delivery.

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3)  If you have a cycle less than 28 days--

Calculate the date as in (1). Then subtract the extra days from it.
This is your expected date of delivery.
Eg If you have a 26 day cycle; to the date calculated as above minus 2 (28-26=2) days.  

A better estimate of the date of delivery can be obtained by ultrasonography.