How Long Is Each Trimester?

When exactly does each trimester begin and end?
ByJayme Klock
February 28, 2017
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If you’ve tried searching the web to find out when each trimester begins and ends, you’ve probably noticed that there are many answers to this question. Different practitioners may use different markers to determine the start and end dates of each trimester. Some just break it up into three chunks of 13 weeks, but most add on an extra week for a grand total of 40 weeks of pregnancy. Here’s the most common breakdown, along with official weeks from the American Congress of Obstetricians and Gynecologists:

First Trimester: 0 weeks–13 and 6/7 weeks (Months 1–3)
Starts with the first day of your last menstrual period (LMP) and ends with the last day of the 13th week thereafter. During your last period (which will be your last for a while), your uterus is preparing for ovulation and your body is gearing up for pregnancy, so technically, you can start counting from this point.

Second Trimester: 14 and 0/7 weeks–27 and 6/7 weeks (Months 4 –7)
Starts at the beginning of the 14th week after your LMP and lasts through the 27th week of pregnancy.

Third Trimester: 28 and 0/7 weeks– 40 and 6/7 weeks (Months 7–9)
Starts at the beginning of the 28th week after your LMP and ends with labor.

Still confused? Use our Due Date Calculator tool help pinpoint which trimester you’re in based on your estimated due date.

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