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How is Pregnancy Due Date Calculated?

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

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 86 Comments
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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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Neo Jul 14, 2017 03:29:55 AM ET

I found out i'm pregnant and my due date was on the 16th july 2015..i slept with my man on the 01st of november 2014..when did i conceive?

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moleboheng Jul 2, 2017 01:34:58 AM ET

I slept with my ex on 1st september 2013 and then i slept with my boyfriend the rest of september, so i'm not sure who the father is. my due date was in june 2014.

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Lara Jun 30, 2017 01:09:06 PM ET

My lmp is march 17,2017. sexed with my ex on march 21 then other man on march 29. who is the father of my baby? i'm so confused.

Guest Nov 18, 2017 10:03:21 AM ET

When is your due date?

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Lisa Jun 4, 2017 04:30:58 PM ET

I had a fling with a guy the whole month of november 2014 and a little into december, protected sex every time except once the condom fell off inside. i don't recall missing a period as i was due for it on december 14-15. him and i stopped having sex in the beginning-ish/middle-ish of december maybe before i can't remember. anyway my ex fiance and i rekindled our relationship on january 13th 2015 and i was on my period(so that was potentially two cycles since i've last slept with guy one) i took my best friend at the time out for her birthday on january 13th and later that night i got a random text from my ex fiance wanting to see me. so i gave him oral as i was on my period. we started exclusively hanging out after that having unprotected sex every time with him finishing in me everytime like rabbits. valentine's day came and we were supposed to go see 50 shades of grey when it came out but weather was terrible on valentine's day and i was expecting my period that week. so the following friday came i decided to take a test and boom pregnant. i went to the doctor and was given the due date of october 21st and delivered october 27th at 41 weeks. is it possible guy 1 is the dad?? everything leads to no except i look at my son now and he has dark eyes like guy one???

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Guest Jun 2, 2017 04:29:09 PM ET

If i had sex on 28th october and 18th november, but my due date is 21st july. is there any chance i got pregnant 18th nov?

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Jasmine May 22, 2017 10:32:04 AM ET

If i had unprotected sex on nov 1 2016 and due date august 1 , would that person be the father?

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Karissa Apr 17, 2017 08:49:05 AM ET

I had sex on the 19th of march 2017. what day is my due date?

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Guest Apr 10, 2017 07:00:27 AM ET

My last period was on march 10th, 2017. i had sex with my b/f on march 14th, 2017. my next period was supposed to come on apr. 8th, 2017, but it didn't come. can i be pregnant or what? i have no signs of pregnancy.

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