How to Calculate Due Date
The formula most often used to calculate a baby's due date is based on a calculation developed by a German obstetrician Dr. Franz Naegele back in the 1850s. Hence the formula is often referred to as Naegele's Rule.
Dr. Naegele's studies concluded that the average length of time between conception and a baby's birth was 266 days. His studies also showed that the average ovulation occurred 14 days from the first day of a woman's last menstrual period. Therefore your baby's due date can be estimated by adding 280 days (14 + 266) from the first day of your last menstrual period.
Or, using a shorthand method, you can also estimate your baby's birth date by adding 7 days to the first day of your last period and then subtracting 3 months from the result to get the month and day of the due date.
For example, if the first day of your last menstrual period was April 7th, your estimated due date would be January 14th (4/7 + 7 days = 4/14, minus 3 calendar months = 1/14).
40 Weeks Calculator
Use the following to calculator to add or subtract 40 weeks (280 days, or 9.20547 months) to or from a given date.
Due Date Based on Weeks and Days Along
If you know how many weeks and days you are along, but not the due date, the following calculator will estimate your due date based on weeks and days along.