How is Pregnancy Due Date Calculated?by Elizabeth Keefe | August 21, 2008
You may have visited your obstetrician for your first prenatal visit expecting to find out when you due date is. One of the questions she will ask you is, "What was the date of the first day of your last menstrual period?" and "How long does your cycle normally last". She then miraculously comes up with an expected due date of your baby.
So how is your doctor calculating your due date based on this information? A typical pregnancy lasts 280 days or 40 weeks. Pregnancy due date is calculated from the date of the first day of your last period (LMP) plus 280 days.
A woman's average menstrual cycle is 28 days from the beginning of one period to the beginning of the next period. For a woman who have clockwork-like periods and her cycle is always 28 days, it can be assumed that she ovulated on day 14. This is usually when conception will occur, (but is not always the case with every woman). So does this mean your doctor is counting the two weeks before your typical ovulation date as part of your pregnancy? Yes is does! But unless your doctor can pinpoint the exact moment you conceived your baby, this is the most accurate way to calculate when to expect your little bundle of joy.
On to the Calculation!
So let's say that a woman's last menstrual period was on June 4. It would be assumed that she ovulated on June 18, (14 days later). So her due date would be calculated like this:
Calculation: LMP Date + 280 days = 11 Pregnancy Due Date
A Real example: June 4 + 280 days = March 11 Pregnancy Due Date
What about Irregular cycles?
However, not all women have the same cycle length. How is due date calculated if a woman's cycle is not normally 28 days? In situations such as this where a woman's cycle is longer or shorter, the doctor must determine an adjusted LMP date by either adding (for cycles longer than 28 days) or subtracting (for cycles shorter than 28 days) the remainder number of days to 28 days.
Adjusted LMP Calculation for Longer Cycles
For example, let's say another woman's cycle is normally 31 days (3 days longer than the average cycle). It would be assumed that she conceived 3 days later than a woman who has a 28 day cycle, so the doctor will add 3 days to her due date using the following formula:
Calculation: LMP date + (31 days - 28 days = 3 days) + 280 days = Pregnancy Due Date
A Real example: June 4 + (31 days - 28 days) + 280 days = March 14
In this example, a woman had her last menstrual period on June 4. Her cycle is 31 days so you add 3 days from her LMP and then add 280 days to arrive at her pregnancy due date.
Adjusted LMP Calculation for Shorter Cycles
Another example for a woman whose cycle is normally 25 days long. It will be assumed that she conceived 3 days sooner than if she had a cycle 28 days long:
Calculation: LMP date - (25 days - 28 days) + 280 Days = Pregnancy Due Date
A Real example: June 4 - (25 days - 28 days) + 280 days = March 8
In this example, a woman had her last menstrual period on June 4. Her cycle is only 25 days so you subtract 3 days from her LMP and then add 280 days to arrive at her pregnancy due date.
Confused? We understand, so we made it easy and created a tool for you to make it easier to calculate your pregnancy due date.
Enter the first day of your last period to calculate your due date.
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