According to Nagel's rule, how do you calculate the due date?

The correct method to calculate the due date according to Nagel's rule is to add 7 days to the first day of the last menstrual period (LMP) and then count back 3 months. This approach aligns with the common practice for estimating due dates based on the average length of pregnancy, which is about 40 weeks or 280 days from the first day of the last menstrual period.

This method incorporates the notion that pregnancy is typically counted from the LMP, thus when counting back 3 months, you're aligning the calculation with this starting point. The addition of 7 days is a necessary adjustment to account for the initial month of pregnancy (since you are starting your count from the first day of the LMP) and finalize the calculation to provide an estimated delivery date.

Other methods might miscalculate the due date by either incorrect additions or subtractions, leading to a misunderstanding of the 40-week gestation period. Understanding this calculation helps with the proper planning for prenatal care and expectations for the pregnancy timeline.