Understanding Due Dates: Why LMP Calculations May Vary

There are a few reasons why your due date calculated from your last menstrual period (LMP) might be different from the date your doctor gave you.

* Your menstrual cycle may not be exactly 28 days. The average menstrual cycle is 28 days, but some women have cycles that are shorter or longer. If your cycle is shorter than 28 days, your due date will be earlier than the date calculated using the standard 28-day cycle. If your cycle is longer than 28 days, your due date will be later.

* You may have ovulated later in your cycle than usual. Ovulation usually occurs 14 days before the start of your period. However, some women ovulate later in their cycle, which can lead to a later due date.

* Your doctor may use a different method to calculate your due date. Your doctor may use the Naegele rule, which is the most common method for calculating due dates. The Naegele rule adds 280 days (40 weeks) to the first day of your last period. However, some doctors use other methods, such as the McDonald rule or the Budin rule, which can produce slightly different due dates.

If you are concerned about your due date, talk to your doctor.