How are the three trimesters of pregnancy divided?

Pregnancy is counted as 40 weeks from the first day of your last menstrual period and split into three trimesters. The first trimester runs from week 1 to the end of week 13, the second trimester from week 14 to the end of week 27, and the third trimester from week 28 until birth.

Can I calculate my trimester if I only know my due date?

Yes. If you know your estimated due date, the calculator works backwards to find how many weeks pregnant you are today and which trimester that falls into. You can also calculate from your last menstrual period or your conception date.

🤰 Pregnancy

Trimester Calculator 2026

Find out exactly which trimester you are in, how many weeks pregnant you are and your estimated due date. Calculate from your last period, conception date or due date — and see what to expect at each stage of pregnancy.

Calculate from

First day of your last menstrual period

The first day of your most recent period (LMP)

Calculate as of

Defaults to today — change it to check a past or future date

Average cycle length (optional)

Only used with the last-period method. Default is 28 days.

You are in the

Enter a date to see your trimester

📅 How far along --

🌸 Current trimester --

🍼 Estimated conception date --

🎯 Estimated due date --

⏳ Days remaining --

➡️ Next trimester starts --

📋 The Three Trimesters at a Glance

Pregnancy is counted as 40 weeks from the first day of your last period.
Trimester	Weeks	Roughly
First trimester	Week 1 – 13	Months 1–3
Second trimester	Week 14 – 27	Months 4–6
Third trimester	Week 28 – birth	Months 7–9

Note: Different sources draw the lines slightly differently (some end the second trimester at week 26 or 28). This calculator uses the common 13 / 27 / 40-week split used by most health services.

🍼 What to Expect in Each Trimester

🌱 First trimester (weeks 1–13)

The baby's major organs, brain and heartbeat develop. You may feel very tired and experience morning sickness, tender breasts and food aversions. This is the most important time to take folic acid and avoid alcohol and smoking.

🌸 Second trimester (weeks 14–27)

Often called the "honeymoon" phase — sickness usually eases and energy returns. You'll likely feel the first baby movements, start to show, and can find out the baby's sex at the mid-pregnancy scan around week 20.

🤰 Third trimester (weeks 28–40)

The baby gains weight quickly and gets into position for birth. You may have backache, heartburn, swelling and trouble sleeping. Antenatal visits become more frequent as you get ready to meet your baby.

📌 Key Milestones by Week

🫀 Heartbeat detectable~ Week 6

🩺 Dating / NT scanWeeks 11–14

🌸 Second trimester beginsWeek 14

👶 First movements ("quickening")Weeks 16–22

🖼️ Anomaly scanWeeks 18–21

🤰 Third trimester beginsWeek 28

✅ Considered full termWeek 37

🎯 Estimated due dateWeek 40

Which Trimester Am I In?

A full-term pregnancy lasts about 40 weeks, counted from the first day of your last menstrual period (LMP), and is divided into three trimesters. The first trimester covers weeks 1 to 13, the second weeks 14 to 27, and the third from week 28 until your baby is born. This calculator works out your current week and trimester from your last period, conception date or estimated due date.

How is the due date worked out?

The standard method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last period. If you have longer or shorter cycles than the average 28 days, ovulation — and therefore your due date — shifts slightly, which is why you can enter your cycle length. If you know your conception date, the due date is roughly 266 days later. Remember that only about 1 in 20 babies actually arrives on the exact due date.

Why trimesters matter

Each trimester brings different changes for you and your baby, different tests and scans, and different things to watch out for. Knowing which trimester you're in helps you understand what's happening now and what to prepare for next. Always pair this with the personalised care of your own midwife or doctor.

Frequently Asked Questions

How are the three trimesters divided? +

Pregnancy is counted as 40 weeks from your last period. The first trimester runs weeks 1–13, the second weeks 14–27, and the third from week 28 until birth.

Which trimester am I in? +

It depends on how many weeks pregnant you are: 0–13 weeks is the first trimester, 14–27 weeks the second, and 28+ weeks the third. Enter your date above to see your exact week and trimester.

How is my due date calculated? +

Naegele's rule adds 280 days (40 weeks) to the first day of your last period. From a conception date it's about 266 days. Only around 5% of babies arrive on the exact due date.

What happens in each trimester? +

The first trimester is when organs form (and sickness is common), the second is usually more comfortable with the first movements, and in the third the baby grows quickly and gets ready for birth.

Can I calculate from only my due date? +

Yes. Choose the "Due date" method and the calculator works backwards to find your current week and trimester. You can also calculate from your last period or conception date.

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Medical note: This calculator gives general estimates only and is not medical advice. Dates from a calculation can differ from a dating ultrasound — always follow the guidance of your own doctor or midwife.