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How is Pregnancy Due Date Calculated?

by Elizabeth Keefe
23 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 23 Comments
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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.

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divya Jul 2, 2014 02:54:47 AM ET

I am still confused. when is my due date? my last period was on may 7th.

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aminat adams Jun 21, 2014 03:27:28 AM ET

I saw my last menstruation on 5th february 2014. the doctor said my due date is in november. which month did i get pregnant?

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charlie Jun 14, 2014 06:24:22 PM ET

My last period was the last week of may. i had unprotected sex with my significant other june 8th, 9th and 10th. could i be pregnant, and if so how far am i? i took an at home test. it was negative, but it might have been to soon to tell. i need help.

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polo jr Jun 14, 2014 03:47:00 PM ET

My due date december 14. what month did i get pregnant? february or march?

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rita marie Jun 10, 2014 02:51:40 AM ET

I had my period the ending of april, for 4 days. it was a short period. i normally have it for 7 days. then, during the next 2 week's i was feeling nothing but lower back pain cramps. now for this week i still feel it, but not so much. it's more like a constipation. what is my problem? i had another period on may 26th and it ended on may 28th. what is wrong with me? did i get pregnant or have a miscarriage?

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tshegofatso May 31, 2014 02:07:36 PM ET

I had sex on the 24th of march when did i conceive the baby?

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Guest Jul 11, 2013 04:17:01 PM ET

My period was june 10,2013 its now the 11th i'mi only miss a period if i'm preganant im having craming sore breast sleeping a lot what's going on

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rosie Jul 10, 2013 05:23:12 AM ET

My period ended on 21st june, i had sex with my nd on 24th. could i be pregnant?

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mommy of 3 ♡ Jul 3, 2013 11:08:37 PM ET

I just had a baby march 20th 2013 and i had sex on the 24th & 25 th of april and then a bunch of times after that i did a test on the 10th of june and it came back that i am pregnant again but i never got a period after giving birth when would i be due then? and how would the drs know when im due??

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