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How is Pregnancy Due Date Calculated?

by Elizabeth Keefe | August 21, 2008 12:00 AM
71 Comments


How is Pregnancy Due Date Calculated?

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.

Month Day Year

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Showing 1 - 10 out of 67 Comments
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diana Apr 8, 2015 12:30:50 AM ET

If i had sex on october 4 2014, when is my due date?

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maribel Apr 1, 2015 12:58:52 PM ET

I had sex with my b/f on february 21, 2015, and i got my period february 28th through march 6th. i don't know when i'm getting my period because i had it for both months. so when am i getting it?

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luchy Mar 26, 2015 09:48:26 AM ET

Saw my period last on feb 13th. made love with my hubby, and tested positive. how many weeks pregnant am i, and when will my due date be?

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monique Mar 20, 2015 11:45:31 AM ET

My last period was on february 13, 2015. i had sex with my b/f on february 10th and march 6, 2015. my next period was supposed to come on march 12, 2015, but it didn't come. can i be pregnant or what? i have no signs of pregnancy. somebody please tell me something!! i'm going to the doctor today.

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mama de un pollito Mar 18, 2015 07:29:43 PM ET

My last period was in the last days of sept 2014. i sleep with two guys every two weeks. i'm 23 weeks pregnant. i would like to know who my baby's dad is.

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tasha Mar 16, 2015 01:46:14 PM ET

I had sex on june 1 2014, and took a pregnancy test two weeks later, and it was positive. i'm due february 1st, but july 18th was my first ultrasound, and i was 11 weeks. did i get pregnant before june 1st?

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mary Mar 15, 2015 11:04:44 PM ET

I am due aug 31st. i slept with my boyfriend every day besides dec 10th. i was with a different guy. what is the possibility that it's not my boyfriend's? please help.

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suzy Mar 12, 2015 03:18:10 PM ET

My last period was on the 15th to the 20th of february 2014. i slept with 4 lads. one lad on the 24th of february, the second lad on the 26th feb, the third on the 28th, and the other on the 1st march. am i pregnant?

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Sakshi Mar 11, 2015 01:17:08 AM ET

My last period was on the 26th of jan 2015. how many weeks pregnant am i?

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brooke Mar 9, 2015 11:12:24 AM ET

I had sex at the end of may with an ex, and doctors say i got pregnant on june 12th. my due date is march 5th. is he the father?

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