Trimester Calculator
Find out which trimester you're in — and what that actually means. See what's happening with your baby, your body and your scan schedule right now.
Which trimester are you in?
Pick the date you know. The calculator does the rest.
Cycle length adjustment (for irregular or long/short cycles)
What's happening in your trimester
👶 Baby is…
🤰 You might feel…
🏥 Clinical priorities
What to book this trimester
Your whole pregnancy at a glance
How trimesters work
Pregnancy is divided into three roughly equal stages — the first, second and third trimester. Each one lasts about thirteen weeks, and each one is defined by a distinct phase of development for your baby and a distinct experience for you. The word "trimester" simply means three-month period.
The divisions aren't arbitrary. The first trimester (up to 13 weeks) is when all major organs form and the miscarriage risk is highest. The second trimester (14 to 27 weeks) is when you typically feel your best — nausea eases, energy returns, and you begin to feel movement. The third trimester (28 weeks to birth) is when your baby gains most of their weight and prepares for life outside the womb.
Clinicians occasionally draw the lines in slightly different places — some put the first/second boundary at 12 weeks, others at 14. This calculator uses 13 and 27 weeks as the transition points, which matches the most widely accepted UK convention.
Why the divisions matter
Every pregnancy scan and clinical milestone is tied to a specific trimester. The dating scan falls in the first; the anomaly scan defines the middle of the second; growth scans dominate the third.
Knowing your trimester isn't just a label — it tells you what care you should be receiving, what symptoms are normal, and what to plan for next.
The three trimesters at a glance
How each trimester is defined, what's happening inside, and the clinical priorities that come with it.
| Trimester | Weeks | Length | Defining feature |
|---|---|---|---|
| First | 0 – 13 | 13 weeks | All major organs form; miscarriage risk highest |
| Second | 14 – 27 | 14 weeks | The "golden" trimester; anomaly scan at 18–22 weeks |
| Third | 28 – birth | ~13 weeks | Rapid growth; preparing for labour and delivery |
Frequently asked questions
When does each trimester start and end?
This calculator uses the most common UK convention: the first trimester runs from 0 to 13 weeks, the second from 14 to 27 weeks, and the third from 28 weeks until birth. A few clinicians draw the first-to-second boundary at 12 weeks instead of 13, but the difference is minor and doesn't affect clinical care.
Why aren't the trimesters all the same length?
They're close — 13, 14 and ~13 weeks respectively — but not identical. The divisions are clinical, not mathematical. They mark biological transitions rather than dividing pregnancy into three equal parts. The second trimester is slightly longer because it captures the middle phase where development continues steadily without the dramatic changes of the first or the heavy weight gain of the third.
Which trimester is the most difficult?
It varies by person, but surveys generally rank the first as the most physically challenging (nausea, fatigue, heightened miscarriage risk) and the third as the most uncomfortable (size, sleep disruption, back pain). The second trimester is often called the "golden trimester" because most women feel their best — nausea eases, energy returns, and the baby is still small enough to be comfortable.
Does a dating scan change my trimester?
Only if the scan shifts your gestational age by more than five days. A dating scan measures your baby directly and is more accurate than any date-based method. If the scan re-dates you, your trimester boundaries move with it. For most women, the scan confirms what the calculator already shows.
What if I'm on a border — say, 13 weeks 6 days?
You're still in the first trimester until you hit 14 weeks 0 days. The transition happens at the start of the next week, not partway through. On 14w 0d, you officially enter the second trimester.
Do trimesters work the same for twin pregnancies?
Yes, the weeks and divisions are identical. However, twin pregnancies usually deliver earlier — typically 36 to 37 weeks for dichorionic twins — so the third trimester may be shorter in practice. The clinical schedule is also more intensive, with growth scans every two to four weeks from 20 weeks onwards.
Book the right scan for your trimester
First trimester? Dating or early scan. Second? Anomaly or gender. Third? Growth or presentation. Same-week appointments at our Kensington clinic.