You're counting down now. Two trimesters behind you, one to go, and your due date is suddenly close enough to plan around. So which week actually starts the third trimester?
The short version: week 28. That's the line most doctors and pregnancy apps use.
But the edges get fuzzy. Some sources say 27, a few say 29, and the weeks-versus-months math throws almost everyone off at least once. So let's lay the whole timeline out clearly.
The short answer
The third trimester starts at week 28 of pregnancy and runs until birth, usually around week 40. So it spans weeks 28 to 40, the final 12 to 13 weeks.
Week 28 is the standard medical cutoff. The NHS, the American Pregnancy Association, and most clinics all use it. A few sources put the start at 27 weeks and a couple say 29, so a small difference in what you read is normal and nothing to worry about.
Pregnancy weeks by trimester
A full pregnancy lasts about 40 weeks, counted from the first day of your last period. Those weeks divide into three trimesters of roughly 13 weeks each.
Here's how the whole thing breaks down.
| Trimester | Weeks | Months | What's happening |
|---|---|---|---|
| First | 1 to 12 | 1 to 3 | Major organs form and morning sickness often peaks; the higher-risk early window passes |
| Second | 13 to 27 | 4 to 6 | Energy usually returns and your bump starts to show. You feel the first kicks |
| Third | 28 to 40 | 7 to 9 | Baby gains fat quickly, lungs mature, and the body prepares for labour |
You'll see slightly different boundaries depending on the source. Some put the first trimester through week 13 and start the second at week 14. Others end the second at week 26. The week-28 start for the third trimester is the most common version by far.
Why does the third trimester start at week 28?
Trimester just means "three months." Split 40 weeks into three roughly equal parts and you get blocks of about 13 weeks, which lands the boundaries near week 13 and week 27. So week 28 is where the final third begins.
There's a medical reason the 28-week mark sticks, too. It's a meaningful point for the baby's development and survival odds. A baby born at 28 weeks has a much stronger chance of doing well than one born just a few weeks earlier, partly because the lungs and brain have matured more by then.
One thing trips people up constantly: the difference between "28 weeks" and "the 28th week." When an app says you're 28 weeks, it means 28 completed weeks plus a few days, written like 28+3. The third trimester starts the moment you tick over to 28 weeks and 0 days.
How to find your third trimester start date
You can work it out from one date: the first day of your last period.
Week 28 lands 196 days after that day (28 weeks times 7). Counting the other direction, it's about 12 weeks before your due date.
Here's a worked example.
Say your last period started on Monday, January 5, 2026. Add 196 days and you reach Monday, July 20, 2026. That's day one of your third trimester. Your due date, 40 weeks out, falls on Monday, October 12, 2026.
If you'd rather skip the arithmetic, the pregnancy week calculator does it for you: enter your last period or due date and it returns your current week, your trimester, and how far along you are. To check which weekday a given date falls on, the weekday calculator handles that in one step.
In a spreadsheet, it's a one-liner. Put your last-period date in cell A1:
=A1+196 → third trimester starts
=A1+280 → estimated due date
One quick clarification, since this site is mostly about week numbers: pregnancy weeks and ISO week numbers measure different things. Pregnancy weeks count from your last period and run from 1 to about 40. ISO week numbers count the weeks of the calendar year, 1 to 52 or 53. For the calendar week number of a date, use the week calculator instead.
What happens during the third trimester
This stretch is mostly about growth. The baby roughly triples in weight and fills out the space it's got.
A few markers along the way:
- Around week 28, the baby weighs about 2 to 2.5 pounds and can open its eyes. Your antenatal checkups usually become more frequent from here.
- By week 32, weight gain speeds up and the baby starts "practice" breathing movements.
- At week 37, you're considered term (more on that just below).
- Week 40 is your due date, though plenty of babies arrive a week or two on either side of it.
You'll likely feel it physically: more pelvic pressure, backache, shortness of breath, and broken sleep as the bump gets bigger. All normal, and all worth mentioning at your checkups if anything feels off.
When are you considered full term?
From 37 weeks, a pregnancy is considered term, meaning the baby is generally ready to be born. Doctors split that final window into more precise labels.
| Label | Weeks |
|---|---|
| Early term | 37 to 38 weeks |
| Full term | 39 to 40 weeks |
| Late term | 41 weeks |
| Post-term | 42 weeks and beyond |
So "full term" in the strict sense means 39 to 40 weeks, even though people often say 37 weeks casually. If you reach 42 weeks, your provider will usually talk through your options.
FAQ
Is the third trimester weeks 28 to 40?
Yes. In the standard medical timeline the third trimester runs from week 28 until birth, which is usually around week 40. A few sources start it at 27 or 29 weeks, but 28 to 40 is the common version.
How many weeks is the third trimester?
About 12 to 13 weeks, from week 28 to week 40. It can run longer if you go past your due date, which is why many people feel it's the longest trimester.
Is 27 weeks second or third trimester?
For most sources, week 27 is the last week of the second trimester, with the third starting at week 28. A few guides do count 27 weeks as the start of the third trimester, so it depends on which one you're reading.
What month does the third trimester start?
Month 7. Week 28 falls roughly 6.5 months into your pregnancy, so the third trimester covers months 7, 8, and 9.
Can my third trimester start earlier or later than week 28?
The week-28 mark is fixed by your gestational age, so it doesn't shift from person to person. What changes is the calendar date it lands on, and that depends entirely on the first day of your last period or your due date.
Quick reference
- The third trimester is weeks 28 to 40, months 7 to 9, the final 12 to 13 weeks.
- Week 28 starts 196 days after your last period, or about 12 weeks before your due date.
- Term begins at 37 weeks; full term in the strict sense is 39 to 40 weeks.
- Find your exact week and trimester with the pregnancy week calculator, and check any calendar date with the weekday calculator.