Does the 1st Trimester end at 12 or 13 weeks?

Re: Does the 1st Trimester end at 12 or 13 weeks?

• MrsV092207

  January 2009

  I think it's 13.. because 40 divided by 3 is 13.3333  etc

  Kristy
  
  

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• PuppyMommy

  January 2009

  I have heard different things, too. I know on The Bump, the girls go by 13 w 3 days (at least I think), so that's what I have been going by.

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• JCM052707

  January 2009

  Yes, 13 weeks 3 days is more accurate.  But most people don't realize they're pregnant for 10 months, not 9, so they think 12 weeks = 3 months, so 1st tri is over.  But each tri is actually just a bit longer than 3 months.

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• RachB

  January 2009

  My OB says it's 13 weeks but everyone thinks differently. It's very confusing. For instance, when I log-in to the bump, it says I'm on week 13 but my ticker still says 12. So, which is it? I dunno.
  

  

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• robynlesley member

  January 2009

  Most books I read say the 1st Trimester ends after you've completed 12 weeks.  So that would be you're in the 2nd Tri when you're at 13 weeks.

  However, the Bump girls consider 2nd Tri to be 13weeks, 3 days?which basically divides the 40 weeks into 3.

  If you're only at 12 weeks today, you're still in the 1st Tri.
  

  
  
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• PuppyMommy

  January 2009

  RachB:

  My OB says it's 13 weeks but everyone thinks differently. It's very confusing. For instance, when I log-in to the bump, it says I'm on week 13 but my ticker still says 12. So, which is it? I dunno.
  

  My understanding of this is that technically you are in your 13th week.  You haven't yet completed 13 weeks, but by being 12 w and a few days pregnant, you are in your 13th week. 

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• b.heather member

  January 2009

  I wondered this too! Here is a nice breakdown that shows how really confusing it can be.

   

  The three basic ways to calculate trimesters

  There are basically three ways of dividing up a pregnancy into the three trimesters; they are by: Development,   Gestation,   Conception. They give different dates for when the second trimester begins and for when the third trimester begins. Your health care provider might prefer to use the Development method, while another Midwife or Doctor may use the Gestation method. I am sure they do that just to confuse pregnant women.   Development: This uses actual developmental stages to divide up a pregnancy. From LMP to 12 weeks the embryo develops all the major organs and becomes a fetus. From 12 weeks to 27 weeks the fetus continues developing and reaches viability. From 27 weeks on the fetus finishes development and prepares for delivery. Here the second trimester begins at 12w 0d and the third at 27w 0d. Gestation: With this method you take the 40 weeks of gestation and divide by three. Here the second trimester begins at 13w 3d and the third at 26w 6d. Conception: This method is where you take the 38 weeks of post conception development, divide by three, and add two weeks. Here the second trimester begins at 14w 5d and the third at 27w 3d. No matter what method is used; a pregnancy has a beginning, middle and end. You can use the trimester dates from your Doctor, a book or a website, whichever you feel most comfortable with. https://www.baby2see.com/trimester_calculator.html

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• JCM052707

  January 2009

  PuppyMommy:

  RachB:

  My OB says it's 13 weeks but everyone thinks differently. It's very confusing. For instance, when I log-in to the bump, it says I'm on week 13 but my ticker still says 12. So, which is it? I dunno.
  

  My understanding of this is that technically you are in your 13th week.  You haven't yet completed 13 weeks, but by being 12 w and a few days pregnant, you are in your 13th week. 

  Yes, she is correct.  You are working on week 13 right now.  Think about it, you've already completed week 12 so if you're 12 weeks and 1-6 days , what week does that mean you're on? 13...

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• JCM052707

  January 2009

  robynlesley:

  Most books I read say the 1st Trimester ends after you've completed 12 weeks.  So that would be you're in the 2nd Tri when you're at 13 weeks.

  When you have COMPLETED 12 weeks, you are 12 weeks 0 days..... 12w1d and you are working on your 13th week.

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