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A 10 Month Pregnancy?

Is a pregnancy nine months or ten months long? This is a popular pregnancy debate with many women across message boards and in pregnancy communities today. Most people know that a pregnancy consists of the first, second and third trimester. The definition of a trimester is “A period or term of three months”. If this is the case, why do so many people consider themselves pregnant for ten months?Β  The answer is simple, even if not actually correct. Pregnancies are considered full term at 40 weeks, and many women count four weeks as a month while they are pregnant, making the result a ten month pregnancy. On average there are actually 4.33 weeks in a month, however that a third of a week tends to get lost in the pregnancy shuffle for much of us.

Many women also find it easier, when asked how pregnant they are, to answer in weeks instead of months, because pregnancy is generally a count down. “I am 24 weeks, only 16 weeks to go!” The most general deduction by a bystander would be that the woman is already six months pregnant, when in actuality, per the Gregorian calendar that most of the world lives by, she is only about five and a half months pregnant.

Women who consider themselves 10 months pregnant usually use this form of calculations:

1-4 1
9-12 3
17-20 5
25-28 7
33-36 9

For those that live by the Lunar calendar, this not only makes sense, but is absolutely correct. The Lunar calendar, which is the four weeks it takes for the moon to go from a new moon, to a full moon and back again, would make a pregnancy last for ten months.

For those that live by the Gregorian calendar, and counting by the date you started your last period (let’s say October 15), you would not be one full month pregnant until November 15 and the calculations generally look like this:

November 15 1
January 15 3
March 15 5
May 15 7
July 15 9

However, neither of these calculations is exactly accurate. The most common way to figure out your due date, including the way that most doctors calculate it, is to add nine months and seven days to the start date of your last menstrual period, which would actually be a week before you would have ovulated with a 28-day cycle. Therefore, if you started your LMP on October 15, you due date would be calculated as July 22, not July 15.

November 15 1 4.33
January 15 3 13 (start of second trimester)
March 15 5 21.66
May 15 7 30.33
July 15 9 39

This would make your due date of July 22 be at 40 weeks exactly, or nine months and one week after you started your LMP. This being said, why would someone CHOOSE to be pregnant for ten months?

Another way to look at it is if you consider a pregnancy of 40 weeks to be ten months, and that a year has 52 weeks in it – which you also count four weeks as one month – wouldn’t this mean that your baby would be 13 months old before he turned a year, and you could celebrate his first birthday?

Do you know your baby's birth stone? Birth Flower? How about when you'll be handing over the car keys? Check out our popular Fun Birthday Facts Calculator to discover fun trivia about your baby's birth or due date!

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45 Responses to “A 10 Month Pregnancy?”


  1. Calculating Your Due Date | Early Pregnancy | Baby, Pregnancy, and Parenting at Babies Online
  2. Don't understand dates! - Pregnancy - Second Trimester Forum
  3. 15 weeks pregnant and none the wiser « Rock-a-bye Baby

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