Pregnancy trimesters: the weeks in each, by source
Pregnancy trimesters are the three stages that the roughly 40 weeks of pregnancy are grouped into, counted from the first day of the last period. ACOG, the NHS and the US Office on Women's Health draw the lines between them in slightly different weeks, and your own clinic follows one of them.
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Medically reviewed by Dr Mary Davidson, Reproductive endocrinologist, checked on
The trimesters of pregnancy are a convention for talking about a pregnancy in three parts. ACOG says the 40 weeks of pregnancy are often grouped into three trimesters and counts them from the first day of the last menstrual period, so the first two weeks of the count come before conception (ACOG, "How Your Fetus Grows During Pregnancy", 2024). The Office on Women's Health also counts about 40 weeks from the first day of the last normal period and groups them into three trimesters (Office on Women's Health, "Stages of pregnancy", 2025). Where each trimester begins and ends is where the sources part company.
Trimester weeks: the three ranges side by side
The table sets out the trimester weeks as each source defines them. ACOG gives its lines in weeks and days. The NHS gives no definition in words: its week-by-week guide files weeks 4 to 12 under the first trimester, weeks 13 to 27 under the second and weeks 28 to 41 under the third (NHS, "Week-by-week guide to pregnancy", 2023). The Office on Women's Health uses whole weeks (Office on Women's Health, 2025).
| Source | First | Second | Third |
|---|---|---|---|
| ACOG (US) | Last period to 13 weeks 6 days | 14 weeks 0 days to 27 weeks 6 days | 28 weeks 0 days to 40 weeks 6 days |
| NHS guide (UK) | Weeks 4 to 12 | Weeks 13 to 27 | Weeks 28 to 41 |
| Office on Women's Health (US) | Weeks 1 to 12 | Weeks 13 to 28 | Weeks 29 to 40 |
The NHS guide's pages begin at week 4, so its first trimester section starts there, and they run to week 41, one week past the 40 weeks that ACOG counts to the due date. This site's week pages read "week 13" as 13 weeks and 0 days to 13 weeks and 6 days, and the comparisons below use the same reading.
Patient pages from hospitals and charities draw the lines in more places still. Tommy's, the UK pregnancy charity, gives the third trimester as the start of week 28 (28 weeks and 0 days) to week 40 (Tommy's, "the third trimester", 2025). Jackson Health, a Miami hospital system, gives 0 to 13 weeks, 14 to 26 weeks and 27 to 40 weeks, and says a trimester lasts between 12 and 14 weeks (Jackson Health System, "Understanding each trimester of your pregnancy", 2026).
How long is the first trimester?
By ACOG's definition the first trimester runs from the first day of the last period to 13 weeks and 6 days, which is 14 weeks in all (ACOG, 2024). The NHS and the Office on Women's Health both end it at week 12 and begin the second trimester at week 13. So the answer to "when does the first trimester end" is the end of week 12 or the end of week 13, depending on whose lines your clinic uses. The first trimester weeks, with the tests and appointments that fall in them, are set out in the guide to the first trimester.
When does the second trimester start?
The second trimester starts at week 13 in the NHS guide and on the Office on Women's Health page, and at 14 weeks and 0 days in ACOG's definitions. Under ACOG's lines all of week 13 is still in the first trimester, so the answers sit a week apart. The scans and tests usually offered in these months are in the guide to the second trimester.
When does the third trimester start?
The third trimester starts at 28 weeks and 0 days in ACOG's definitions, at week 28 in the NHS guide and in Tommy's, and at week 29 on the Office on Women's Health page. Jackson Health starts it at week 27. If you have read both 27 and 28, both are in print; 28 is the line ACOG and the NHS share. The guide to the third trimester covers the appointments and growth in the final weeks.
Why the sources disagree
Three things produce the different lines.
- Counting completed weeks or the week you are in. ACOG writes gestational age in weeks and days, so "13 weeks and 6 days" is exact (ACOG, 2024). A page that says "week 13" may mean the 13th week of pregnancy or 13 completed weeks, and the NHS and Office on Women's Health pages do not say which. That alone can move a line by a week.
- Whole weeks or weeks and days. The Office on Women's Health lists whole weeks: 1 to 12, 13 to 28 and 29 to 40 (Office on Women's Health, 2025). Whole-week ranges have to put each boundary week on one side or the other.
- What the page is for. The NHS guide is organized as a page for each week, so its trimester sections are groups of pages (NHS, 2023). A hospital overview such as Jackson Health's divides 40 weeks into three similar blocks (Jackson Health System, 2026).
None of these lines changes anything clinical. Appointments, scans and screening windows are given in weeks, not trimesters, and your care team follows its own schedule.
What happens in each trimester, in brief
ACOG gives each trimester a one-line description. The first is the time when fertilization and major organ development take place. The second is a time of rapid growth and development. The third is when the fetus gains weight and the organs mature so that they are ready to work after birth (ACOG, "How Your Fetus Grows During Pregnancy", 2024). The Office on Women's Health adds markers along the way: by 12 weeks the external sex organs show whether the baby is a boy or a girl, by 20 weeks the baby can hear and swallow, and at 39 weeks the baby is considered full term (Office on Women's Health, "Stages of pregnancy", 2025).
How long is a trimester?
About three months. ACOG describes a trimester as a 3-month period (ACOG, 2024). Measured in weeks, ACOG's own lines give 14 weeks for the first trimester, 14 weeks for the second and 13 weeks for the third; that count is our arithmetic from its definitions, not a figure ACOG states. Jackson Health puts it as between 12 and 14 weeks (Jackson Health System, 2026).
Every week, by trimester
This site's week pages follow the NHS grouping. Each one describes that week as the NHS, ACOG and the Office on Women's Health describe it, and the hub for all of them is pregnancy week by week.
First trimester: 4 weeks pregnant, 5 weeks pregnant, 6 weeks pregnant, 7 weeks pregnant, 8 weeks pregnant, 9 weeks pregnant, 10 weeks pregnant, 11 weeks pregnant, 12 weeks pregnant.
Second trimester: 13 weeks pregnant, 14 weeks pregnant, 15 weeks pregnant, 16 weeks pregnant, 17 weeks pregnant, 18 weeks pregnant, 19 weeks pregnant, 20 weeks pregnant, 21 weeks pregnant, 22 weeks pregnant, 23 weeks pregnant, 24 weeks pregnant, 25 weeks pregnant, 26 weeks pregnant, 27 weeks pregnant.
Third trimester: 28 weeks pregnant, 29 weeks pregnant, 30 weeks pregnant, 31 weeks pregnant, 32 weeks pregnant, 33 weeks pregnant, 34 weeks pregnant, 35 weeks pregnant, 36 weeks pregnant, 37 weeks pregnant, 38 weeks pregnant, 39 weeks pregnant, 40 weeks pregnant.
Weeks 12, 13, 27 and 28 each have a section on where the trimester line falls, because those are the weeks the sources place differently.
Months and trimesters
Because a trimester is described as about three months, people often convert weeks into months to find their place. None of the three sources gives a conversion table: they count in weeks. The guide to pregnancy weeks to months does the arithmetic and labels it as ours, including why 40 weeks comes to a little over nine calendar months.
Find your trimester from your dates
The pregnancy week calculator works as a trimester calculator: it gives your gestational age in weeks and days from your dates, such as the first day of your last period or a due date, and shows the trimester that age falls in. The due date calculator works out the due date itself. If a scan has given you a different date, the date your clinic uses is the one that counts.
This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.
Sources
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy (FAQ156). ACOG, 2024.
- NHS Best Start in Life. Week-by-week guide to pregnancy. nhs.uk, 2023.
- Office on Women's Health, US Department of Health and Human Services. Stages of pregnancy. womenshealth.gov, 2025.
- Tommy's. Everything you need to know about the third trimester (weeks 28 to 40). tommys.org, 2025.
- Jackson Health System. Understanding each trimester of your pregnancy. maternity.jacksonhealth.org, 2026.
Frequently asked questions
How long is a trimester?
About three months, or 13 to 14 weeks. ACOG describes a trimester as a 3-month period, and by its own week lines the first and second trimesters each span 14 weeks and the third 13 weeks (ACOG, 2024). That week count is our arithmetic from ACOG's definitions.
How many weeks are in each trimester?
It depends on the source. By ACOG's definitions the first trimester runs to 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks to 40 weeks and 6 days (ACOG, 2024). The NHS week-by-week guide groups weeks 13 to 27 as the second trimester and 28 to 41 as the third (NHS, 2023), and the US Office on Women's Health uses weeks 1 to 12, 13 to 28 and 29 to 40 (2025).
When does the second trimester start?
At week 13 in the NHS guide and on the Office on Women's Health page, and at 14 weeks and 0 days in ACOG's definitions, so under ACOG's lines all of week 13 is still in the first trimester. Your own clinic will tell you which line it uses.
When does the third trimester start?
At 28 weeks and 0 days by ACOG's definitions, at week 28 in the NHS guide, and at week 29 on the Office on Women's Health page. Some hospital pages start it at week 27. Your own clinic will tell you which line it uses.