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The Guessing Game: Due Dates Can Vary by 5 Weeks, So Says New Study

By Aela Mass |

Due Dates Can Vary by 5 Weeks

Calculating due dates is a crazy “science.” I’ll never forget the first time that my RE (reproductive endocrinologist) explained to me how due dates are determined – I always assumed it was linked to the date of conception. But no.

As many of you likely know, it’s calculated from the first day of your last period – which is totally whack because that would mean you are technically pregnant during a time (a few days to two-ish weeks) when you are definitely not pregnant: before you’ve even conceived.

How did it ever come to be that doctors – scientific, logical professionals – agreed to calculate due dates this way? The answer is rooted back to the Bible. Yep, that’s right. This “science” is linked to the Bible. A botanist from the 1700s came up with this calculating method from “evidence in the Bible that [indicates] human gestation lasts approximately 10 lunar months.” (Source)

It’s bothered me ever since my RE told me about this. Not for any other reason than it seems pretty sketchy for doctors to base charts and calculations on anything but actual facts, and I was disappointed that they’ve seemingly all agreed to continue this for centuries.

So I wasn’t that surprised when I came across a Discovery News article entitled, “Pregnancy Due Dates Can Vary by 5 Weeks.” Surely, anything based on a clearly and obviously uncertain way of calculation is going to be just that: uncertain.

The researchers and doctors involved in these findings have said that the “average” pregnancy is 38 weeks and 2 days – but from the time of ovulation, not the first day of the last period. They used the ovulation date and not the period date because they believed that is a more accurate way of calculating due dates.

Ya think?

While 38 weeks and 2 days was the average time of gestation from the day of ovulation, the doctors also discovered that “the length of the pregnancies varied by as much as 37 days.” That’s just over 5 weeks, people.

So even using what’s considered a more accurate way of calculating a due date (from the time of ovulation), the due dates for the pregnancies of the women involved in this study were still totally random.


I guess that’s why no one has been anxiously trying to change the age-old way of calculating due dates…

::How close to your due date did you deliver?::

Source: Discovery News
Photo: iStockphoto

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About Aela Mass


Aela Mass

Aela Mass is a lesbian writer and editor living the dream on Martha's Vineyard with her wife, Sara, and their dog, Darla. She miscarried her twins at 17 weeks and has undergone numerous IVF, FET, and IUI cycles. Her writing has appeared in The Huffington Post among other publications. For more of her work, visit her blog Two Moms Make a Right. Read bio and latest posts → Read Aela's latest posts →

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3 thoughts on “The Guessing Game: Due Dates Can Vary by 5 Weeks, So Says New Study

  1. April says:

    Due dates are a silly concept, and are taken way too literally! My baby was born at exactly 42 weeks. We went into labor all on our own and had a normal, healthy, all natural (hospital) birth. This seems to surprise a lot of people, and that’s a shame. 42 weeks was exactly the right about of time for both of us.

  2. TBerry says:

    I was 1 week past my due date and I had a dating ultrasound. Since some tests accuracy are based on the stage of pregnancy this sounds like it really should be changed, or make dating ultrasounds a standard practice.

  3. Kari says:

    My husband and I were using Natural Family Planning, so I even know the day I conceived. My son was born at 34 weeks and 5 days. May daughter was born at 35 weeks and 5 days.

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