Expected Date of Delivery
The expected date of delivery is calculated by taking the first day of the last periods. The expected date of delivery will be after 40 weeks from the first day of last periods. The calculation will be by adding 9 months and 7 days to the date of first day of last periods.
Suppose the first day of last periods was December 15th, then by adding 9 months, the date will be September 15th. Add 7 days to that, and then the date of delivery expected will fall on 22nd of September.