Pregnancy Trimesters
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Updated 20 October 2026
Quick answer: Pregnancy trimesters split the roughly 40 weeks of pregnancy into three stages. By ACOG's count, the first trimester runs 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. NHS guides start the second trimester at week 13.
This hub explains what happens in each trimester and links to every Momi guide for that stage. Find your week with How Many Weeks Pregnant Am I? or browse pregnancy week by week.
Pregnancy trimesters at a glance
| First trimester | Second trimester | Third trimester | |
|---|---|---|---|
| Weeks (ACOG) | 1 to 13 + 6 days | 14 to 27 + 6 days | 28 to birth |
| Weeks (NHS guides) | 1 to 12 | 13 to 27 | 28 to 40+ |
| Roughly | Months 1–3 | Months 4–6 | Months 7–9 |
| Your baby | Organs start to form | Grows fast; you feel movements | Gains weight; lungs mature |
| You may notice | Nausea, tiredness, sore breasts | More energy, growing bump | Backache, poor sleep, Braxton Hicks |
| Key checks | Booking visit, dating scan, screening | Anatomy scan, glucose test if offered | More frequent checks, movements, birth plan |
Why do sources differ by a week? It's rounding. See when does the second trimester start and is pregnancy 9 or 10 months.
First trimester (weeks 1–13)
The first trimester starts on the first day of your last period, about 2 weeks before conception. Your baby's major organs begin to form, and by the end of it your baby is around the size of a lemon.
- First trimester symptoms
- When do pregnancy symptoms start?
- Morning sickness
- First trimester screening
- Weeks: week 4 · week 8 · week 12
Second trimester (weeks 14–27)
Often the most comfortable stage. Nausea usually eases, your bump shows, and you'll probably feel your baby move for the first time, often between 16 and 24 weeks.
- Why the second trimester is called the honeymoon period
- When does the second trimester start?
- Baby size by week
- Can you fly while pregnant?
- Weeks: week 16 · week 20 · week 24
Third trimester (week 28 to birth)
Your baby gains most of their weight, and you'll see your midwife or doctor more often. Get to know your baby's pattern of movements and learn the signs of labor. ACOG defines full term as 39 weeks to 40 weeks + 6 days.
- Third trimester guide
- How to track fetal movement
- Braxton Hicks vs real contractions
- When to go to the hospital for contractions
- Weeks: week 28 · week 36 · week 40
Free tools for every trimester
- Due date calculator
- Kick counter for the third trimester
- Contraction timer for labor
Planning across all three trimesters
- Pregnancy appointment schedule
- Pregnancy to-do list week by week
- Pregnancy weight gain week by week
- Pregnancy week by week chart
Track every trimester with Momi
Momi shows your week, trimester tips and your baby's size, with a diary, bump photos, visits, kick counter and contraction timer in one app. Download Momi free on the App Store or Google Play. Free with ads; optional Premium.
Sources
- ACOG, How Your Fetus Grows During Pregnancy
- NHS, Your pregnancy week by week
- ACOG, Prenatal Genetic Screening Tests
- NHS, 12-week scan
- ACOG, Committee Opinion 579: Definition of Term Pregnancy
Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.
FAQ
What is the third trimester of pregnancy?
The third trimester is the last stage of pregnancy, from 28 weeks until your baby is born. Your baby gains most of their weight, you'll have more frequent checks, and you'll start counting movements and preparing for labor. Full term is 39 weeks.
What does first trimester mean?
The first trimester is the first stage of pregnancy, counted from the first day of your last period to the end of week 13 (ACOG). Your baby's organs start to form, and many people have nausea, tiredness and sore breasts.
What is first trimester screening?
It's optional testing, usually between about 10 and 14 weeks, that estimates the chance of conditions such as Down syndrome. It can include a blood test and an ultrasound measurement. A screening result shows a chance, not a diagnosis.
Does the second trimester start at 12 weeks?
Not quite. ACOG starts the second trimester at 14 weeks. NHS week-by-week guides start it at week 13. Week 12 is still the first trimester in both, though it's when many people have their dating scan.
Does the third trimester start at 27 weeks?
ACOG and the NHS both start the third trimester at 28 weeks. Week 27 is the last week of the second trimester. Some books round differently, which is why you'll see both numbers.
Related guides
Track this in Momi
Momi is a free pregnancy tracker app for iPhone and Android that follows pregnancy week by week, estimates the due date, and keeps a pregnancy journal, weight log, kick counter and contraction timer in one place.
