How to Calculate Expected Date of Delivery for Pregnancy

On an average, expected date of delivery is calculated by adding 280 days to the first day of the last menstural period for a cycle duration of 28 days. This is 10 lunar months. A lunar months is 28 days. It is also 40 weeks. Because the number of days in a standard Christian calender varies from 28 to 31, only an approximate value of 9 months and 7 days is given. Assuming 30-day months, then it would be 9 months and 10 days.