Estimated date of delivery

The estimated date of delivery (EDD), also known as expected date of confinement, and estimated due date or simply due date, is a term describing the estimated delivery date for a pregnant woman. Normal pregnancies last between 37 and 42 weeks.
Origins of the term.
Confinement is a traditional term referring to the period of pregnancy when an upper-class, noble, or royal woman would withdraw from society in medieval and tudor times and be confined to their rooms with midwives, ladies-in-waiting and female family members only to attend them. This was believed to calm the mother and reduce the risk of premature delivery. Except in threatened pregnancies (for example, in pre-eclampsia), "lying-in" or bedrest is no longer a part of antenatal care.
Estimation methods.
Due date estimation basically follows two steps:
Estimation of gestational age.
According to American Congress of Obstetricians and Gynecologists, the main methods to calculate gestational age are:
Estimation of gestational age at childbirth.
Childbirth on average occurs at a gestational age of 280 days (40 weeks), which is therefore often used as a standard estimation for individual pregnancies. However, alternative durations as well as more individualized methods have also been suggested. There is in any case considerable variation among individual pregnancies.
Variability.
Given that these gestation lengths are only estimates of an average, it is helpful to consider gestation time as a range of dates rather than as a single "due date". The median is merely a guideline for the day at which half of all births occur earlier, and half of all births occur later. Births rarely occur "on" a due date, but they are clustered "around" due dates. A study of singleton live births in the US came to the result that childbirth has a standard deviation of 14 days when gestational age is estimated by first trimester ultrasound, and 16 days when estimated directly by last menstrual period.
Naegele's rule.
"Naegele's rule" is a standard way of calculating the due date for a pregnancy when assuming a gestational age of 280 days at childbirth. The rule estimates the expected date of delivery (EDD) by adding a year, subtracting three months, and adding seven days to the origin of gestational age. The result is approximately 280 days (40 weeks) from the start of the last menstrual period. Another method is by adding 9 months and 7 days to the first day of the last menstrual period.
Naegele's rule is named after Franz Karl Naegele, the German obstetrician who devised the rule. Naegele was born July 12, 1778, in Düsseldorf, Germany. In 1806, Naegele became ordinary professor and director of the lying-in hospital in Heidelberg. His "Lehrbuch der Geburtshilfe", published in 1830 for midwives, enjoyed a successful 14 editions.
Here's the formula to calculate your Estimated Due Date using Naegele's rule :<br>
"Date of Last Menstrual Period + 7 Days + 9 Calendar Months = Date of Estimated Date of Delivery"
"Example:"<br>
LMP = 8 May 2009
280 days past the start of the last menstrual period is found by checking the day of the week of the LMP and adjusting the calculated date to land on the same day of the week. Using the example above, 8 May 2009 is a Friday. The calculated date (15 February) is a Monday; adjusting to the closest Friday produces 12 February, which is exactly 280 days past 8 May. The calculation method does not always result in 280 days because not all calendar months are the same length; it does not account for leap years.
Mobile apps.
Mobile apps essentially always give consistent estimations compared to each other and correct for leap year, while pregnancy wheels made of paper can differ from each other by 7 days and generally do not correct for leap year.