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How is Pregnancy Due Date Calculated?

by Elizabeth Keefe | August 21, 2008
35 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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Comments


Showing 1 - 10 out of 32 Comments
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Amber Dec 12, 2014 02:48:10 PM ET

If i had sex may 08,2014, when is my due date? when did i conceive?

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Guest Dec 10, 2014 07:31:49 PM ET

I had sex with someone june 17th, but had my period june 20th. than had sex with my normal bf on july 6th and 7th. my doctor says my due date is march 30th. who would most likely be the father? i didn't find out i was pregnant until august because i didn't have a july period, and in june, i thought i was ok because i had my period.

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veev Nov 12, 2014 11:54:56 AM ET

I had my lmp on the 21st of july 2014. when is my due date? this can be confusing.

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Guest Nov 1, 2014 04:37:34 AM ET

My due date is feb 26, 2015. i last had sex on may 21st. is my due date right?

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Jessie Oct 17, 2014 10:58:10 PM ET

Well i'm 6 months pregnant. i had my last period on may 1st, so my due date is feb 5th. however, i had an ultrasound say i'm due on jan 31, 2015. i'm confused about when i got pregnant.

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Guest Oct 11, 2014 07:59:40 AM ET

I had my last period on the 5th of september. before that, my period came around august 13th to 14th rounding up 21 days to the 5th of september. my cycle hasn't normally been 28 days. it's mainly 25 or 26 days, until now. i have not seen my period and no headache or vomiting, but i'm noticing some changes in my breasts. could this be a pregnancy sign?

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Red hot Aug 17, 2014 08:46:56 PM ET

I've had sex on may the 25th, and i got pregnant. when is my due date?

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Guest Aug 10, 2014 08:19:48 AM ET

When i'm adding from 1 month to the next, what date do i add it from? is it from the first day of my last period? what i mean is say is is it from the 10th of this month to the 10th of next month? how do i add it up?

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Muñecaa Jul 27, 2014 02:40:08 PM ET

I just had a baby on feb 28. my last period was on march 4, and it's already late - july 14. how far a long am i? help please.

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abe Jul 18, 2014 11:57:20 PM ET

My girlfriend and i had sex june 17 , 18, and 19 she's telling me she's pregnant. could the baby be mine? i need help.

Chaparra Jul 26, 2014 08:08:12 PM ET

That's crazy to ask

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