🤰 Pregnancy Trimester Calculator
By ToolNimba Health Team · Reviewed by ToolNimba Medical Review, maternal health content review · Updated 2026-06-19
This calculator gives an estimate only and is not a diagnosis or medical advice. It assumes a regular 28-day cycle with ovulation around day 14, which is not true for everyone, so an early ultrasound dating scan is more accurate. Trimester boundaries are conventions and can be drawn slightly differently. Always confirm your dates and any concerns with your doctor or midwife.
| Trimester | Weeks | Date range |
|---|---|---|
| First trimester | 0 to 13 | - |
| Second trimester | 14 to 27 | - |
| Third trimester | 28 to birth | - |
This pregnancy trimester calculator tells you exactly which trimester you are in, how many weeks and days pregnant you are today, and the calendar date range of each trimester. Enter the first day of your last menstrual period (LMP), or switch to enter your estimated due date instead. The tool counts gestational age from the LMP, the same way clinicians do, and maps it onto the first, second, and third trimesters.
What is the Pregnancy Trimester Calculator?
A trimester is one of the three stages doctors use to divide a pregnancy. The most common convention measures from the first day of your last menstrual period (LMP): the first trimester runs from week 0 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. Each stage covers a distinct phase of development, so knowing which one you are in helps you anticipate appointments, tests, and how you are likely to feel.
Gestational age is the number of weeks and days since the first day of your last period, written as something like 16 weeks 3 days. It is counted from the LMP rather than from conception because most people know when their period started but not the exact day they conceived, which is usually about two weeks later. This is why gestational age runs roughly two weeks ahead of the actual age of the embryo, and why a full-term pregnancy is described as 40 weeks even though fertilisation happened around week 2.
To turn dates into a trimester this tool first finds your LMP. If you entered a due date instead, it works backward by subtracting 280 days (40 weeks), since the standard Naegele rule sets the due date that far after the LMP. From the LMP it then counts the days to today to get your gestational age, picks the matching trimester, and lays out the start and end date of all three stages so you can see the whole timeline at a glance. Remember the result is an estimate centred on a likely window, not a fixed schedule.
When to use it
- Quickly answering "what trimester am I in?" without counting weeks by hand.
- Checking exactly how many weeks and days pregnant you are today.
- Seeing the calendar dates each trimester starts and ends, to plan scans and appointments.
- Working backward from an estimated due date when you do not remember your last period clearly.
How to use the Pregnancy Trimester Calculator
- Choose whether you are entering your last period (LMP) or your estimated due date.
- Pick that date in the date field.
- Read off your current trimester, your gestational age in weeks and days, and your estimated due date.
- Use the date-range table to see when each trimester begins and ends.
Formula & method
Worked examples
Your last period started on 1 January 2026 and today is 1 May 2026.
- Days from 1 January to 1 May 2026 = 120 days
- Gestational age = 120 ÷ 7 = 17 weeks 1 day
- Week 17 falls in the range 14 to 27
- Therefore you are in the second trimester
Result: Second trimester, 17 weeks 1 day pregnant
You only know your estimated due date: 7 October 2026, and today is 8 July 2026.
- Work back to the LMP: 7 October 2026 − 280 days = 31 December 2025
- Days from 31 December 2025 to 8 July 2026 = 189 days
- Gestational age = 189 ÷ 7 = 27 weeks 0 days
- Week 27 falls in the range 14 to 27, so it is still the second trimester
Result: Second trimester, 27 weeks 0 days pregnant
Pregnancy trimesters by gestational age (weeks counted from LMP)
| Trimester | Weeks | What happens |
|---|---|---|
| First trimester | Week 0 to 13 | Conception, organ formation, morning sickness, highest miscarriage risk |
| Second trimester | Week 14 to 27 | Bump shows, first movements, anatomy scan, often the most comfortable stage |
| Third trimester | Week 28 to birth | Rapid growth, more frequent checks, full term from week 37 |
Common pregnancy milestones by week
| Gestational age | Milestone |
|---|---|
| Week 12 to 13 | End of the first trimester, dating scan often done |
| Week 18 to 22 | Anatomy (mid-pregnancy) ultrasound |
| Week 24 | Generally considered the limit of viability |
| Week 28 | Start of the third trimester |
| Week 37 | Pregnancy is considered full term |
| Week 40 | Estimated due date |
Common mistakes to avoid
- Counting trimesters from conception. Trimesters here are measured from the first day of your last period, not from conception. Counting from conception puts you about two weeks behind and can place you in the wrong trimester near a boundary.
- Assuming each trimester is exactly three months. The three stages are not equal calendar thirds. The first is about 13 weeks, the second about 14, and the third runs from week 28 until birth, which can be 12 weeks or more.
- Using the day your period ended. The calculation needs the first day of your last period, not the day it finished. Using the end date shifts every trimester boundary several days too late.
- Treating boundaries as exact for an irregular cycle. If your cycles are long, short, or irregular, ovulation does not fall on day 14, so an LMP-based trimester can be off by days. An early dating scan is the more reliable guide.
Glossary
- Trimester
- One of the three stages of pregnancy: the first (weeks 0 to 13), second (weeks 14 to 27), and third (week 28 to birth).
- Gestational age
- How far along a pregnancy is, measured in weeks and days from the first day of the last menstrual period.
- LMP
- Last menstrual period, the first day of your most recent period, used as the starting point for counting weeks.
- Naegele rule
- The standard estimate that sets the due date 280 days (40 weeks) after the first day of the last menstrual period.
- Full term
- A pregnancy that has reached 37 weeks of gestation, when the baby is considered ready for birth.
- Dating scan
- An early ultrasound that measures the baby to estimate gestational age, usually more accurate than an LMP estimate.
Frequently asked questions
What trimester am I in?
Count the weeks from the first day of your last period. Weeks 0 to 13 are the first trimester, weeks 14 to 27 are the second, and week 28 until birth is the third. Enter your date above and the calculator shows your trimester instantly.
How many weeks are in each trimester?
The first trimester covers about 13 weeks, the second about 14 weeks (14 to 27), and the third runs from week 28 until birth, usually around 12 to 14 weeks. The stages are not exactly equal, so they do not line up with neat calendar months.
When does the second trimester start?
The second trimester begins at the start of week 14 and ends at the close of week 27. It is often called the most comfortable stage, when early nausea has eased and the anatomy scan is usually carried out around weeks 18 to 22.
When does the third trimester start?
The third trimester starts at week 28 and lasts until birth. A pregnancy is considered full term from 37 weeks, and most births happen between 37 and 42 weeks, so the third trimester can vary in length.
Can I use my due date instead of my last period?
Yes. Switch the option to "estimated due date" and enter it. The calculator works backward by subtracting 280 days to find your LMP, then counts your weeks and trimester from there.
Is this calculator a substitute for seeing a doctor?
No. It gives an estimate to help you plan, not medical advice or a diagnosis. Trimester boundaries are conventions and dates assume a regular cycle. Always confirm your dates and any concerns with a qualified doctor or midwife.
Sources
- Stages of Pregnancy , U.S. Office on Women’s Health
- Methods for Estimating the Due Date (Committee Opinion) , American College of Obstetricians and Gynecologists