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.
The standard trimester breakdown
| Trimester | Weeks | Roughly |
|---|---|---|
| First | Weeks 1–13 | Months 1–3 |
| Second | Weeks 14–27 | Months 4–6 |
| Third | Week 28 – birth | Months 7–9 |
First trimester (weeks 1–13)
The foundation phase: all major organs form, the heartbeat starts, and hormones surge — which is why fatigue and nausea peak now. Most first scans and screening tests happen toward the end of this trimester.
Second trimester (weeks 14–27)
Often called the "golden trimester": energy tends to return, the bump appears, and somewhere around week 16–25 you'll likely feel the first kicks. The 20-week anatomy scan — and the halfway milestone — land here.
Third trimester (week 28 to birth)
The growth sprint. Your baby roughly triples in weight, practises breathing, and settles head-down. Appointments get more frequent, and it's the time to prepare for labor — including learning how to time contractions and the 5-1-1 rule.
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
Is week 13 or week 14 the start of the second trimester?
Most providers count weeks 1-13 as the first trimester and start the second at week 14. Some sources draw the line a week earlier or later — your provider's convention is the one that matters for your care.
How many weeks is full term?
39-40 weeks is full term. Birth between 37 and 42 weeks is considered within the normal window.
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.