Pregnancy Trimester Calculator
Map estimated pregnancy weeks and days to trimester boundaries. Use your recorded clinician due date or a basic last-period estimate, and see why some guides start the second trimester one week earlier.
A trimester label is a calendar grouping. It does not measure fetal growth, confirm pregnancy or decide when a test, trip or treatment is appropriate.
Use the calculatorMap Your Pregnancy Stage and Transition Dates
Follow the dating and trimester convention used by your care team.
Your trimester timeline will appear here
Enter a dating input and assessment date. The result shows estimated weeks, the selected trimester label and calendar transitions.
Use recorded dating
An assigned clinician EDD keeps the timeline aligned with the pregnancy notes.
See the boundary difference
At 13+0 to 13+6, ACOG and the NHS week guide use different trimester labels for the same calendar age.
Keep labels in context
The tool cannot assess symptoms, fetal size, scan findings or clinical appointment eligibility.
Your trimester calendar estimate
Calendar position on a 40-week reference
Medical disclaimer: This is an educational pregnancy calendar estimate, not a diagnosis, fetal-growth assessment or clinical dating service. It does not confirm pregnancy, predict birth or determine screening, treatment or travel eligibility. Use the due date and appointment plan recorded by your maternity or fertility care team. Do not delay seeking advice about symptoms because of a trimester label.
Trimester boundaries under your selected convention
| Stage or marker | Weeks + days | Starts | Ends |
|---|
Compare labels without changing pregnancy age
| Convention | Label on assessment date | Second trimester begins | Third trimester begins |
|---|
Why 13+0 and 14+0 can have different labels
Use completed weeks plus remaining days. Boundary conventions describe groups, not biological switches.
| Convention | First trimester | Second trimester | Third trimester |
|---|---|---|---|
| ACOG | 0+0 to 13+6 | 14+0 to 27+6 | 28+0 until birth |
| NHS week-guide grouping | Before 13+0 | 13+0 to 27+6 | 28+0 until birth |
Check ACOG trimester definitions · Check NHS week-guide groups. We apply the guide’s 13-to-27-week grouping at 13+0; this does not claim every NHS service uses the same convention.
Read 13 weeks and 6 days precisely
Fictional LMP 8 July 2026, assessed 13 October: 97 days = 13+6. ACOG labels this first trimester; the NHS guide labels it second. The ACOG transition is 14 October.
Map the 28+0 boundary
Fictional recorded EDD 5 January 2027, assessed 13 October 2026. Subtracting 280 days derives 31 March 2026 as 0+0; assessment age is 28+0, third trimester under both conventions.
Third trimester does not stop at 40+0
Fictional LMP 1 January 2026, assessed 13 October: 40+5. The 8 October due-date marker is five days earlier. The trimester label remains third; no birth or care decision is predicted.
Separate dating, labels and clinical decisions
A calendar week is useful context; it cannot answer every pregnancy question.
13+6 is not 14+0
13+6 means 13 completed weeks and six additional days, or 97 days. The following day is 14+0. A phrase such as “in your 14th week” can be used informally for 13+0 onward; this calculator always shows completed weeks to avoid that ambiguity.
A derived origin is not a conception date
With clinician EDD input, subtracting 280 days creates a calendar origin for gestational-age arithmetic. It does not identify the actual LMP, fertilisation or implantation date. Basic LMP dating also counts from before conception usually occurs.
Symptoms do not switch on at a boundary
A trimester transition does not promise nausea relief, a particular energy level, fetal movement, baby size or fetal position. The result reflects dates and definitions, not individual development or wellbeing.
Use clinical dating when it has been assigned
Do not replace the EDD in your notes because a simple LMP calculation differs. Ultrasound and clinical circumstances may inform the recorded date. For IVF or uncertain periods, ask the care team which EDD to use.
| Displayed item | Meaning here | Limit |
|---|---|---|
| Trimester start | Dating origin + 91 or 98 days, then +196 days | Not a symptom threshold or confirmation of organ development. |
| 40-week marker | Dating origin +280 days, or the entered clinician EDD | Not a predicted delivery day or a fixed third-trimester end. |
| Calendar scale | Elapsed days on a 280-day reference, visually capped at the marker | Not percentage fetal growth, readiness for birth or pregnancy completion. |
| Assessment date | The date you chose for the age calculation | Future-date projections do not describe your current stage. |
Appointments use more than the trimester name
Screening and care plans use gestational age, individual history and local protocols. This tool deliberately provides no automatic test deadlines, travel clearance or treatment schedule. Ask the maternity team about the plan recorded for you.
Helpful resources
Related pregnancy and health tools, followed by references for calendar dating and trimester definitions.
Useful resources
References help verify definitions and arithmetic; they do not endorse or clinically validate this calculator. Baby Growth is a post-birth measurement tool, not a fetal-growth assessment.
Questions about trimester boundaries and dates
Week notation, dating inputs, differing labels and what the timeline cannot establish.
Why do some guides start the second trimester at 13 weeks and others at 14?
They use different grouping conventions. This calculator offers the NHS week-guide grouping from 13+0 and ACOG definitions from 14+0. Both use 28+0 for the third trimester. The same calendar age can therefore have a different label during week 13.
What does 13+6 mean?
It means 13 completed weeks plus six additional days, or 97 days from the dating origin. The next day is 14+0. The calculator uses completed weeks and remaining days rather than ambiguous ordinal-week wording.
Does changing the convention change my due date?
No. It changes the first-to-second-trimester grouping only. Gestational age, the dating origin and the estimated due-date marker stay the same.
Which input should I use if my clinician has given me a due date?
Use the recorded clinician EDD. The tool maps that date to calendar weeks and boundaries; it does not revise clinical dating or replace ultrasound and individual assessment.
Is EDD minus 280 days my actual last period?
Not necessarily. It is a derived calendar origin for gestational-age arithmetic. It does not identify your actual period, conception or implantation date.
Does basic LMP mode adjust for cycle length or IVF?
No. It adds 280 days to the LMP without cycle-length, ovulation or IVF adjustments. Uncertain periods, irregular cycles and fertility treatment should use the dating assigned by the care team.
Does third trimester end at 40 weeks?
No. It continues until birth. The 40-week date is an estimated due-date marker, not a fixed trimester endpoint or a guaranteed delivery date.
What does the calendar bar measure?
It shows elapsed calendar days on a 280-day reference and is visually capped at the 40-week marker. It does not measure fetal growth, birth readiness or percentage pregnancy completion.
Can I assess a future date?
Yes, within the tool’s 0+0 to 42+0 range. The result is labelled as a future-date projection, not your current stage. Dates outside the range are rejected rather than silently clamped.
Does this calculator save pregnancy dates?
This calculator script uses browser memory and does not upload entries or save them to browser storage. Other website scripts and services are outside the calculator’s scope.