Pregnancy trimesters explained: which weeks are in each?
"When does the second trimester actually start?" is one of the most searched pregnancy questions — and different sources draw the lines slightly differently. Here's the standard breakdown and what each stage brings.
Which trimester am I in?
Enter the date you know and see your exact week and day, your trimester, and how far through the 40 weeks you are. Everything is worked out in your browser and nothing is sent anywhere.
The trimester breakdown, week by week
Most searches for this land on a three-row table. Here is the full mapping instead, so you can find any week without counting.
| Weeks | Month | Trimester |
|---|---|---|
| 1–4 | Month 1 | First |
| 5–8 | Month 2 | First |
| 9–13 | Month 3 | First |
| 14–17 | Month 4 | Second |
| 18–22 | Month 5 | Second |
| 23–27 | Month 6 | Second |
| 28–31 | Month 7 | Third |
| 32–35 | Month 8 | Third |
| 36–40+ | Month 9 | Third |
Summarised: first trimester weeks 1–13, second trimester weeks 14–27, third trimester week 28 until birth. The months are approximate by design, because a pregnancy is 40 weeks, which is a little over nine calendar months, so the two systems never line up exactly.
Why every week number starts before conception
Pregnancy weeks are counted from the first day of your last menstrual period, not from conception. Conception happens around two weeks later, which means weeks 1 and 2 are, strictly speaking, before you were pregnant. It looks odd, but it is the convention every scan, screening and due date is scheduled against, so it is the one worth using. Our guide on how many weeks pregnant you are covers the counting in detail.
First trimester (weeks 1–13)
The foundation phase. All the major organs form, the heartbeat starts around week 6, and hormone levels climb steeply, which is why fatigue and nausea usually peak here rather than later. The first dating scan and early screening tests generally happen toward the end of this trimester, between weeks 8 and 13.
It is also the longest stretch of pregnancy where nothing is visible from the outside, which many people find the hardest part of it.
Second trimester (weeks 14–27)
Often called the golden trimester: energy tends to return, nausea usually eases, and the bump appears. Somewhere between weeks 16 and 25 you will likely feel the first movements, earlier if this is not your first pregnancy. The 20-week anatomy scan lands here, and so does the halfway mark.
This is the trimester most people describe as the one where pregnancy starts to feel real rather than theoretical.
Third trimester (week 28 to birth)
The growth sprint. Your baby roughly triples in weight, practises breathing movements, and usually settles head-down in the final weeks. Appointments become more frequent, sleep becomes harder, and this is when preparing for labor moves up the list, including learning how to time contractions and knowing the 5-1-1 rule for when to go in.
Note that the third trimester does not end at week 40. Anything from week 37 to week 42 is considered normal timing, so the last trimester is the one whose length you cannot know in advance.
Why sources disagree about week 13
You will find charts that put week 13 in the first trimester and others that put it in the second. Both are defensible: 40 weeks does not divide evenly into three, so the boundary has to be drawn somewhere by convention. The most common split, and the one used here and in Lunera, is 1–13 / 14–27 / 28 onward.
A week either way changes nothing medically. If your provider or your notes use a different split, use theirs, because it is the one your care is scheduled against.
What actually changes at a trimester boundary
Nothing, on the day itself. Trimesters are a way of describing phases that blend into each other, not switches that flip. Their practical use is as shorthand: "second trimester" tells a midwife roughly where you are, groups the appointments and tests that cluster together, and gives you a way to think about a long stretch of time in three manageable parts rather than forty weeks.
Lunera tracks your trimester automatically
No need to remember where the lines fall: Lunera shows your current trimester as a badge right under your week — "Week 20 + 3 days · Second Trimester" — and a progress bar of your journey through all 40 weeks.
- The trimester updates itself the day you cross into the next one
- Each week's info includes what's typical for that stage, for you and your baby
- Widgets keep your week and progress on your home screen throughout all three trimesters
Quick answers
What are the trimesters of pregnancy?
Pregnancy is divided into three trimesters: the first covers weeks 1 to 13, the second weeks 14 to 27, and the third runs from week 28 until birth. The split is a convention for describing three phases of a 40-week pregnancy, not a medical boundary.
How many weeks are in each trimester?
Thirteen weeks in the first, fourteen in the second, and thirteen or more in the third, since the third lasts until you give birth. That is why the numbers do not divide evenly: 40 weeks does not split into three equal parts.
Is week 13 or week 14 the start of the second trimester?
Most providers count weeks 1 to 13 as the first trimester and start the second at week 14. Some charts draw the line a week earlier. It makes no medical difference, and your provider's convention is the one that matters for your care.
How do trimesters map to months of pregnancy?
Roughly: months 1 to 3 are the first trimester, months 4 to 6 the second, and months 7 to 9 the third. The mapping is approximate because 40 weeks is a little over nine calendar months, so weeks and months never line up exactly.
Why does week 1 start before conception?
Because pregnancy weeks are counted from the first day of your last menstrual period, and conception happens about two weeks after that. It means weeks 1 and 2 precede conception, but it is the convention your due date, scans and screenings are all scheduled against.
Which trimester am I in?
Find your current week and read it off the table above, or enter your last period, conception date or due date in the calculator on this page and it will tell you your week, day and trimester directly.
How many weeks is full term?
39 to 40 weeks is considered full term. Birth anywhere between 37 and 42 weeks falls within the normal window, which is why the third trimester has no fixed end.
Does anything change on the day I enter a new trimester?
No. Trimesters describe phases that blend into one another rather than switches that flip. They are useful as shorthand for where you are and for grouping the appointments and tests that cluster together.
Track your pregnancy with Lunera
Week-by-week baby development, due date countdown, widgets, and a one-tap contraction timer — free, private, and subscription-free on iPhone.
This guide is for informational and educational purposes only and is not medical advice. Lunera is not a medical device. Always consult your doctor, midwife, or healthcare provider with any questions about your pregnancy or labor.