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How is Pregnancy Due Date Calculated?

by Elizabeth Keefe | August 21, 2008
49 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 45 Comments
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Guest Kay Jan 26, 2015 01:40:35 AM ET

I had unprotected sex with my boyfriend on the 4th of oct 2014. i don't remember the date of my last period. now how many weeks am i pregnant now?

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nobody Jan 23, 2015 09:55:25 PM ET

I had sex on june 19th. my last cycle was on june 12th. my due date is on march 19th. could i have gotten pregnant on the 19th?

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tee Jan 22, 2015 04:51:07 PM ET

I had sex with my boyfriend during the last week of june with protection. on july 8th, the day before my period. i had sex with my ex without protection, and ever since then i was having sex with my ex. then in august, my period never came back. i found out i was 7 weeks on the 26th of august. i'm confused who could possibly be the father of my child.

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Guest Jan 21, 2015 07:45:54 AM ET

I'm 8 months pregnant now, but the problem is that i don't remember having sex in may. i only slept with my boyfriend on june 22nd. my due date is 9 feb. that's what my doc says. please help me count, because i'm confused. how many months am i, because i only had sex around june?

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Tasha Jan 20, 2015 10:30:21 AM ET

I had sex with someone on september 9th, and then sex with my boyfriend after during the same month. my last period was on september 26th. it lasted 4 to 5 days. who am i pregnant for?

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susan Jan 17, 2015 10:32:35 PM ET

If i had sex on june 27th, when is my due date? someone please help me.

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Dominique Jan 12, 2015 02:22:03 PM ET

I had sex on the 14th of dec with someone, and then had sex with my ex on the 15th of dec. i took a test on the 8th of jan with a clear blue test and it said i'm 1-2 weeks pregnant. i also had sex with my ex on the 18th of december, 25th of december and the 26th of december. i'm all mixed up as to when i could have actually conceived? my last period was on the 8th of december. please help.

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alexandria Jan 12, 2015 12:22:45 AM ET

My last period was on july 30th and it ended on august 5th. i slept with 2 guys, one was on the 7th of august, and the one was on the 12th. my doctor said that august 7th was my free time to have sex. she said i didn't not get pregnant on the 7th. she said i got pregnant on the 13th or 14th of august. can somebody tell me if that's true?

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christie Jan 10, 2015 11:34:29 PM ET

Hi. i'm probably 7 months pregnant now.. my last period was around or after the 26th of june. my doctor says my due date is at the end of march. but, i'm confused. can you help me?

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Guest Jan 5, 2015 08:31:23 AM ET

I had sex with my man on the 7th of may, and the doctor says i am due the 29th of jan. is this true?

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