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How is Pregnancy Due Date Calculated?

by Elizabeth Keefe | August 21, 2008 12:00 AM100 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 95 Comments
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Ruth Sep 7, 2018 09:41:01 AM ET

I went for ultrasound scan 07/09/2018 with 35 weeks n my lmp was 07/01/2018. when should i be expecting my baby? please i need the calculation.

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Eunice Acho Jun 23, 2018 01:30:04 PM ET

I saw my period last 26 of oct 2017. when is my due date?

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Lily Mar 22, 2018 12:17:14 PM ET

I had sex on july 1st and july 11th and my due date was march 30th but, now they are telling me it's april 6th. which due date is true?

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Guest Mar 8, 2018 05:20:12 PM ET

How is a dna test done?

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Guest Mar 8, 2018 05:14:12 PM ET

I gave birth on 25th june 2017. which date did i get pregnant?

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Bello Feb 19, 2018 06:06:35 AM ET

I had sex 5th october and 21st october and my due date is july 13. so when did i get pregnant?

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Karen Feb 10, 2018 02:59:14 PM ET

Hi i want to no wen i got pregnant iam 38 weeks and doctor said my due date is february 22, 2018 i had unprotected sex w my boyfriend on may 6 14 and 26 until june i only slept with 2 different men on may 23 n 24 but i sure used condom with them but steel worry is my boyfriend the father

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Keashia Feb 3, 2018 01:36:37 PM ET

Don't remember my last period but, i didn't have one in june. around may 27 my nipples was sore the whole night that i went out with my girls . 4 days before father�s day i found out i was pregnant and july 3 started getting sick and was 6 weeks and 3 days.

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Eliexer masa Dec 11, 2017 12:40:36 PM ET

If a female got pregnant in march or april but u didn't have sex with her since july, are u the father? but she was messing with someone other than u before u were.

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cally Aug 7, 2017 09:03:16 AM ET

My last period was on 31 december 2016 and it was brownish. that was only 1 day. when is my due date?

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