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Pregnancy Trimester Calculator

Find out which trimester of pregnancy you're in and how many weeks remain, based on your due date.

Result

Current Trimester
Second Trimester
Weeks Until Full Term (40 wks)
20

Uses the common trimester convention: weeks 1-12 (first), 13-26 (second), 27-40+ (third), based on a standard 40-week full-term pregnancy. Individual due dates and trimester boundaries can be adjusted slightly by your doctor based on ultrasound dating.

About the Pregnancy Trimester

The Pregnancy Trimester Calculator identifies which trimester a pregnancy falls into and how many weeks remain until the standard 40-week full-term mark, based on a single weeks-pregnant input. It is a quick reference for anyone who knows their current week count and wants the trimester translated automatically.

How It Works

You enter the number of weeks pregnant, from 0 to 42. The tool compares that number against two cutoffs, under 13 weeks is First Trimester, under 27 weeks is Second Trimester, and everything from 27 weeks onward is Third Trimester. It also subtracts the entered week count from 40 to show weeks remaining until full term, never showing a negative number.

trimester = First Trimester if weeksPregnant < 13, Second Trimester if weeksPregnant < 27, otherwise Third Trimester. weeksRemaining = max(40 - weeksPregnant, 0).

Formula & Methodology

To work this out by hand, compare your current week count against the 13 and 27 week boundaries: below 13 is first trimester, 13 through 26 is second trimester, and 27 or higher is third trimester. Then subtract the week count from 40 to find weeks left until the conventional full-term point, treating any result below zero as zero since a pregnancy at or past 40 weeks has reached the full-term mark.

Examples

Nine weeks along

Entering 9 weeks pregnant falls under the 13-week cutoff, so the result is First Trimester, with 40 - 9 = 31 weeks remaining until the full-term mark.

Thirty weeks along

Entering 30 weeks pregnant is at or past the 27-week cutoff, so the result is Third Trimester, with 40 - 30 = 10 weeks remaining until the full-term mark.

Advantages

  • Gives an instant trimester label from a single number, without needing to look up week-range charts manually.
  • Also surfaces weeks remaining until full term in the same result, saving a separate subtraction step.
  • Uses a widely recognized 40-week full-term convention that lines up with common pregnancy tracking references.

Common Mistakes

  • Entering a due-date-based week estimate that has not been updated after an ultrasound adjusted the dating, causing the trimester result to lag the actual clinical estimate.
  • Expecting the tool to account for multiples (twins or more) or medically adjusted due dates, which this calculator's simple week-based formula does not distinguish.
  • Assuming the 13 and 27 week cutoffs are a universal medical standard rather than one common convention among several slightly different ones.

Edge Cases to Watch For

  • Values of 40, 41, or 42 weeks all return zero weeks remaining, since the subtraction is clamped at zero and the calculator does not distinguish post-term timelines beyond that floor.
  • The boundaries used here (13 and 27 weeks) are one common convention; the calculator's own note acknowledges other sources use slightly different week cutoffs for trimester boundaries.
  • The tool takes weeks pregnant directly rather than a due date, so any rounding or estimation in how a person determines their current week count carries through directly into the trimester result.

Common Use Cases

  • Someone who knows their current week count and wants a fast trimester label without consulting a pregnancy app.
  • A person planning trimester-specific appointments or milestones who wants to quickly see how many weeks remain to 40 weeks.
  • Anyone comparing pregnancy tracking sources that use different trimester boundary conventions and wants to see this site's specific cutoffs.
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Why do trimester week ranges vary slightly between different sources?

There's no single universally standardized week cutoff for each trimester boundary - some sources use 12/13 weeks and 26/27 weeks as dividing points, while others round slightly differently, since a 40-week pregnancy doesn't divide perfectly evenly into three exact thirds and different medical and educational sources have settled on slightly different conventional cutoffs.

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Conclusion

This calculator offers a straightforward, convention-based way to translate a week count into a trimester label and a countdown to full term. Because trimester boundaries vary slightly across sources, it works best as a general reference alongside guidance from a doctor using ultrasound-based dating.

References