Which trimester are you in?
A pregnancy of about 40 weeks is split into three trimesters. This trimester calculator tells you which one you are in today, which week of it, and the exact dates each trimester starts and ends. Work from the first day of your last period (LMP), a due date from your midwife or scan, a conception date, or an IVF embryo transfer date.
Sources do not draw the lines in quite the same place, so the calculator shows the definition it is using and lets you switch between the American College of Obstetricians and Gynecologists (ACOG) and the NHS. It is information only, not medical advice, and runs entirely in your browser.
How to use the Trimester Calculator
- Choose what you know: last period, due date, conception date or IVF transfer.
- Enter the date. For a last period, enter your usual cycle length; for IVF, pick a day-3, day-5 or day-6 embryo.
- Pick ACOG or NHS trimester boundaries.
- Read your trimester and week, then the table of start and end dates. Copy dates puts them on your clipboard.
Trimester Calculator features
Four starting points
Last period (with cycle length), due date, conception and IVF day 3, 5 or 6 — the same rules as the due date calculator.
Two definitions
ACOG (2nd trimester from 14 weeks) or NHS (from 13 weeks), with both shown side by side.
Today on a timeline
A three-part bar marks where you are, and the table says which trimesters are done and when the next begins.
How the trimesters are defined
Pregnancy is counted from the first day of your last period, about two weeks before conception, so “14 weeks” means 14 weeks since that day. ACOG defines the first trimester as the first day of the LMP to 13 weeks and 6 days, the second as 14 weeks 0 days to 27 weeks 6 days and the third as 28 weeks 0 days to 40 weeks 6 days (ACOG FAQ “How Your Fetus Grows During Pregnancy”, updated January 2024, reviewed March 2026).
The NHS week-by-week guide groups its pages differently: the first-trimester pages run to week 12, weeks 13 to 27 are the second trimester and weeks 28 to 41 the third. So in the UK you may read that you are in the second trimester at 13 weeks, while a US source says 14. Both agree that the third trimester starts at 28 weeks. Many apps and books use one or the other, which is why charts differ by a week.
| Trimester | ACOG (US) | NHS (UK) |
|---|---|---|
| First | 0w 0d – 13w 6d | up to 12w 6d |
| Second | 14w 0d – 27w 6d | 13w 0d – 27w 6d |
| Third | 28w 0d – birth | 28w 0d – birth |
How the dates are worked out
The calculator first finds the equivalent first day of your last period. A due date minus 280 days gives it; a conception date minus 14 days; a day-5 IVF transfer minus 19 days (day 3: 17 days, day 6: 20 days). With a cycle longer or shorter than 28 days, the dates move by the difference, because ovulation moves with it. Each trimester then starts a fixed number of days later — day 98 (14 weeks) or day 91 (13 weeks) for the second trimester and day 196 (28 weeks) for the third — and the due date is day 280.
Worked example: a last period on 17 July 2026 gives a second trimester from 23 October 2026 (ACOG) or 16 October 2026 (NHS), a third trimester from 29 January 2027 and a due date of 23 April 2027.
What each trimester usually brings
- First: most of the baby’s organs form; tiredness and sickness are common. A dating scan usually happens around 11 to 14 weeks.
- Second: many people feel better and notice the first movements; the mid-pregnancy anatomy scan is around 18 to 22 weeks.
- Third: the baby gains weight and gets ready for birth. Babies born from 37 weeks are term; most arrive between 37 and 42 weeks.
Related calculators
For a full week-by-week calendar and scan dates use the pregnancy due date calculator. To turn weeks into months, use the pregnancy month calculator; to see how big the baby is now, baby size by week.
Trimester Calculator FAQ
When does the second trimester start?
At 14 weeks 0 days by ACOG’s definition, or at 13 weeks in the NHS week-by-week guide. Choose the definition your clinic or app uses.
When does the third trimester start?
At 28 weeks 0 days — ACOG and the NHS agree. It lasts until birth, usually between 37 and 42 weeks.
How long is each trimester?
With ACOG’s definition the first and second are 14 weeks each and the third about 12 weeks to the due date; with the NHS grouping the first is 13 weeks and the second 15.
Can I use the trimester calculator by due date?
Yes. Choose “My due date” and enter the date your midwife or scan gave you; the calculator counts back 280 days to set every trimester date.
How does the trimester calculator work for IVF?
Choose “My IVF embryo transfer date” and the embryo’s age. A day-5 blastocyst transfer is treated as 2 weeks 5 days of pregnancy on the transfer day, as clinics count it.
Why does my app say a different week?
Apps differ on whether “week 14” means 14 completed weeks or the 14th week, and on the trimester definition. The table shows weeks and days so you can compare exactly.