Estimate pregnancy trimester using clear inputs and transparent formulas, with results intended for planning and comparison.
How to use this calculator
For pregnancy trimester calculator, enter Gestational age in weeks using the unit or format beside each field. Select Calculate to apply the formula below, review all returned values together, and use Reset to restore the demonstrated inputs before testing another scenario.
Formula & calculation method
pregnancyTrimester = if(pregnancyWeeks<14,'firstTrimester',if(pregnancyWeeks<28,'secondTrimester','thirdTrimester')); weekIntoTrimester = if(pregnancyWeeks<14,pregnancyWeeks,if(pregnancyWeeks<28,pregnancyWeeks-13,pregnancyWeeks-27))Worked example
For a reproducible pregnancy trimester calculator example, use Gestational age in weeks: 20. The first returned value, calendar trimester grouping, is calculated with if(pregnancyWeeks<14,'firstTrimester',if(pregnancyWeeks<28,'secondTrimester','thirdTrimester')); the other results describe the same scenario. Change one input at a time and keep its unit consistent when comparing scenarios.
Limitations to keep in mind
Pregnancy and fertility dates are estimates only. Cycle variation, ultrasound dating, embryo stage, multiple pregnancy, and clinical findings can change the appropriate schedule.