Pregnancy & Baby 

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Trimesters: When does each one begin?

By definition, the word ‘trimester’ divides a time into three equal parts. However, pregnancy is not neatly divisible by three. If you calculate from the last menstrual period (LMP) with a 28-day cycle, a pregnancy lasts 40 weeks or 10 lunar months. If you calculate from probable conception, again based on a 28-day cycle, you are left with a gestational period of 38 weeks. Neither of these is divisible by three. As a result, you get somewhere in between 12 2/3 weeks and 13 1/3 weeks. Conventionally, it is common to say that the first trimester ends at 12 weeks (12 weeks from the first day of the last period). The second trimester proceeds from 12 to 28 weeks and the third trimester extends from 28 to 40 weeks.

I suppose it's our quest for symmetry that requires that we break time periods into a beginning, middle and end. It also gives doctors neat compartments in which to discuss changes in physiology, nutritional needs, common discomforts and more.

Otherwise, there is no real significance to dividing the pregnancy into three equal parts. It is more usual to speak of weeks (from your last period) rather than trimesters.

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