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When Does the Second Trimester Begin?

The second trimester, of course, is a great mile marker in pregnancy, as your risk of miscarriage drops significantly.  I’ve been anticipating this week for… well, weeks.  I feel calmer, happier, and generally more settled in my pregnancy.  Truthfully, just being able to say, “I’m in my second trimester!” is a great feeling.

So with a smile on my face, I exit the first trimester and begin the second… I think.  Honestly, there’s no hard-and-fast rule on when the second trimester begins; most websites and books have conflicting information.  Some say 12 weeks; others it’s during the 13th week; and even more believe the second trimester starts towards the end of week 14. 

Pregnancy timelines, in general, are pretty tricky.  After all, the moment you conceive, you’re already 2 weeks pregnant.  What?! How?! Well, the week-by-week count begins at the start of your last menstrual period (LMP).  By the time you figure out you’re pregnant, you’re already at 4 to 5 weeks.

There are three basic ways to calculate trimesters:

  • By Development: This view divides the pregnancy by development milestones.  The first trimester consists of the development of all major organs as the embryo becomes a fetus.  Under this viewpoint, the first trimester is from the LMP to 12 weeks; the second trimester is 12 weeks to 27 weeks (when the fetus reaches viability); and the third trimester is from 27 weeks to delivery.
  • By Gestation: This view simply divides 40 weeks of gestation (measured from the date of your LMP) by three; thus, the second trimester begins at 13 weeks and 3 days, and the third trimester begins at 26 weeks and 6 days.
  • By Conception: This view doesn’t include the two weeks after your LMP and divides 38 weeks of gestation by three; thus, the second trimester begins at 14 weeks and 5 days, and the third trimester begins at 27 weeks and 3 days.

Honestly, I’m not very good at math, and it all seems a bit confusing.  I’m opting to see 13 weeks as the second trimester, as it falls between the By Development and By Gestation calculation.  Plus – I just want to be there already!

If you want to see when your trimesters begin according to the By Development, By Gestation, or By Conception views, check out this handy calculator.

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