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. That is 10 lunar months. A lunar month 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. First and second trimesters are each 13 weeks long (91 days). Anytime after that is third trimester.