Health & Fitness

What Is a Trimester?

A trimester is one of the three periods of about three months into which pregnancy is divided. Counting from the first day of the last menstrual period, a common convention is: first trimester, weeks 1 to 13; second trimester, weeks 14 to 27; and third trimester, week 28 until birth, around week 40.

Week Counting

Pregnancy weeks are usually counted from the last menstrual period, about two weeks before conception, which is also how due dates are estimated.

Slight Variations

Some sources place the boundaries a week earlier or later.

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👶 Pregnancy Due Date Calculator Estimate your due date based on your last menstrual period. Calculate → 📆 Week Number Calculator ISO 8601 and US week numbers for any date, and the dates of any week. Calculate →

🔗 Related Terms

Gestational Age Estimated Due Date (EDD) Menstrual Cycle Ovulation

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Last updated: September 29, 2026. This definition is for general educational purposes only. It is not financial, investment, tax, legal, or medical advice, and it does not recommend any product or course of action. For decisions about your own situation, consult a qualified professional.