Pregnancy Trimester Calculator
See which trimester you’re in and the exact date ranges of all three trimesters, based on your last period.
How it works
From the last period: first trimester = weeks 1–13, second = weeks 14–27, third = weeks 28–40 (through the due date).
The roughly 40 weeks of pregnancy are split into three trimesters. The exact cutoffs vary a little between sources — the first/second boundary is placed anywhere from week 12 to 14 — and this calculator uses the common split of weeks 1–13, 14–27 and 28–40. The differences are small and don’t change your care, which your provider guides by your specific dates.
Worked example
For a last period on June 1, 2026: the first trimester runs June 1 to September 6, 2026; the second, September 7 to December 13, 2026; and the third, December 14, 2026 to the March 8, 2027 due date.
Frequently asked questions
Do trimesters have exact boundaries?
Not universally. Sources place the first/second cutoff between weeks 12 and 14. This tool uses weeks 1–13, 14–27 and 28–40. The small differences don’t affect your care — follow your provider’s schedule.
Why does the first trimester get so much attention?
A great deal of early development happens in the first trimester, and it’s when many people feel the strongest symptoms. Your provider schedules specific screenings by trimester — ask what to expect and when.
The third-trimester dates end at 40 weeks — what if I go past?
The table stops at the 40-week due date for simplicity. Pregnancies can continue a week or two beyond and still be normal; your provider monitors any pregnancy that goes past its due date.
Related calculators
Estimates based on standard formulas and average cycles — every pregnancy is different. This is not medical advice; your doctor or midwife, with an ultrasound, gives the accurate dates and guidance.