Due Date Calculator · Practical guide
What Trimester Am I In by Last Period?
Trimesters are just week counts from your last period: 1-13, 14-27, and 28 to delivery at week 40.
At a glance
The Quick Answer
Pregnancy weeks are counted from the first day of your last menstrual period. Weeks 1-13 are the first trimester, weeks 14-27 the second, and week 28 through the due date (week 40) the third. Divide the days since your LMP by 7 to get your current week, then match it to a trimester.
Interactive tool
Due Date Calculator
Enter your own values, then compare the result with the examples below.
Trimester Dates
| Trimester | Start Date | End Date |
|---|---|---|
| First Trimester | -- | -- |
| Second Trimester | -- | -- |
| Third Trimester | -- | -- |
Exact Date Ranges: Worked Example
For a last period starting January 10, 2026 with a 28-day cycle, the calculator above splits the 280-day span at the starts of weeks 14 and 28:
| Trimester | Weeks | Dates |
|---|---|---|
| First | 1-13 | January 10 - April 11, 2026 |
| Second | 14-27 | April 12 - July 18, 2026 |
| Third | 28-40 | July 19 - October 17, 2026 |
What to Expect in Each Trimester
- First (weeks 1-13): conception and implantation; major organs begin forming; heartbeat usually detectable around week 6; dating ultrasound typically between weeks 8 and 12.
- Second (weeks 14-27): early nausea often eases; the detailed anatomy scan happens around weeks 18-22; first movements are usually felt around weeks 16-20.
- Third (weeks 28-40): rapid fetal weight gain and more frequent checkups; full term begins at week 39, and the due date lands at exactly week 40.
Gestational Age vs Fetal Age
Your provider quotes gestational age, counted from the LMP. This calculator also shows fetal age, counted from conception, which runs about two weeks behind. If the fetal age reads lower than the week your provider mentioned, that two-week offset is why.
Frequently Asked Questions
When does the second trimester start?
At week 14, counted from the first day of your last period — about 92 days in. Many parents call it the easiest stretch, as early nausea typically fades and energy returns.
When does the third trimester start?
At week 28. From then until delivery, prenatal visits usually become more frequent, and full term is reached at week 39.
Why do trimester lengths look uneven?
They are not equal thirds: 13, 14, and 13 weeks. The boundaries follow clinical convention tied to development stages, not a simple split of 40 weeks by three.
Can I be a different week than my app says?
Small discrepancies come from rounding and from cycle-length differences. If a dating ultrasound moved your due date, your week count follows the revised date, not the original LMP count.