In theory, the first trimester is the first 3 months; the second trimester is months 4, 5 & 6; and...
In theory, the first trimester is the first 3 months; the second trimester is months 4, 5 & 6; and the third is whatever is left of your pregnancy. But since there is no exact standard for the length of the reference month (Is a month 30 days? Or 31? What about February? Is there an extra day because of a leap year? What about lunar months?), this is the broadest of the three trimester approximations. That is to say, divining the length of a trimester isn't really a very exact science. It varies by a few days, rounded up or down a week, etc. depending on the source. It's all about "approximate" and "roughly." On Pregnancy & Baby, we split the difference with various sources, and came up with the following: First trimester: 0 to 13 weeks Second trimester: 13 to 26 weeks Third trimester: 26 to 42 weeks On our pregnancy calendar, we really drilled down and split the 280 days pretty much into thirds: First trimester: 0 to 93 days Third trimester: 94 to 186 days Third trimester: 186 to 294 days Now, you will notice that we're counting to 42 weeks/294 days. That's because a lot of babies arrive past their official due date. However, most healthcare providers won't allow a pregnancy to progress beyond 14 days "overdue." (Then there's the very unofficial "fourth trimester" -- the first three months postpartum.) No matter how you slice it, 280 days is a long time to be pregnant. We all like to count the days and weeks and months to make it seem just a little bit shorter.

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