When Does the Second Trimester Start? Every Trimester's Dates
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026
Quick answer: When does the second trimester start? By ACOG's count, at 14 weeks (14 weeks 0 days) after the first day of your last period. NHS week-by-week guides start it at week 13. It runs until the end of week 27, and the third trimester starts at 28 weeks. The difference is just how the 40 weeks are rounded.
Not sure what week you're in? Use How Many Weeks Pregnant Am I? or the due date calculator. This guide sets out the trimester dates side by side, converts them to months and explains what changes in each.
Trimester week ranges: ACOG vs NHS
Pregnancy is counted from the first day of your last menstrual period (LMP), so a full-term pregnancy is about 40 weeks. That's split into three trimesters.
| Trimester | ACOG and most US sources | NHS week-by-week guide |
|---|---|---|
| First | Week 1 to 13 weeks 6 days | Weeks 1 to 12 |
| Second | 14 weeks 0 days to 27 weeks 6 days | Weeks 13 to 27 |
| Third | 28 weeks 0 days to birth | Weeks 28 to 40 (and beyond) |
The only real disagreement is week 13. Both systems agree that the third trimester starts at 28 weeks.
Reading week notation: "13 weeks 6 days" (written 13+6) means you've completed 13 full weeks and 6 days. When you're "in week 14", you've completed 13 weeks; when you're "14 weeks pregnant", you've completed 14. This is why apps and websites can seem a week apart. See the pregnancy week by week chart.
When does each trimester end?
- First trimester ends: at 13+6 (ACOG) or the end of week 12 (NHS).
- Second trimester ends: at 27+6, the end of week 27.
- Third trimester ends: when your baby is born. ACOG calls 37 to 38+6 "early term", 39 to 40+6 "full term", 41 weeks "late term" and 42 weeks or later "post-term". The WHO defines a baby born before 37 weeks as preterm.
Trimesters in months
People often ask which month each trimester is. Because 40 weeks is a bit over nine calendar months, any conversion is approximate.
| Trimester | Weeks (ACOG) | Roughly months |
|---|---|---|
| First | 1–13 | Months 1–3 |
| Second | 14–27 | Months 4–6 |
| Third | 28–40+ | Months 7–9 (and a bit) |
For the full explanation of why pregnancy can be called nine or ten months, see is pregnancy 9 or 10 months?.
What changes in each trimester
First trimester (to week 13)
Your baby's organs start to form, and by the end of this trimester all major structures are in place. For you, this is often the hardest stretch: nausea, tiredness, sore breasts and needing to pee more. Your first appointments, blood tests and dating scan usually happen now. Read more about first trimester symptoms.
Second trimester (weeks 14 to 27)
For many people, nausea eases and energy returns, which is why it's sometimes called the "honeymoon" trimester. Your bump starts to show, the anatomy scan happens at around 18 to 21 weeks, and you'll probably feel your baby move for the first time between 16 and 24 weeks. Mayo Clinic notes that backache, skin changes and leg cramps can start. See why the second trimester is called the honeymoon period.
Third trimester (week 28 to birth)
Your baby grows and gains weight fast. You may feel more tired again, get heartburn, swelling, poor sleep and Braxton Hicks contractions. You'll have more frequent checks. From 28 weeks the NHS advises going to sleep on your side, and it's important to get to know your baby's pattern of movements. See the third trimester guide.
Second trimester milestones at a glance
| Week | What usually happens |
|---|---|
| 14 | Second trimester begins (ACOG); nausea often easing |
| 16 | Some people, especially in a second pregnancy, first feel movements |
| 18–21 | Anatomy (20-week) scan |
| 20 | Halfway point of a 40-week pregnancy |
| 24 | Most first-time parents have felt movements by now |
| 24–28 | Glucose test for gestational diabetes if offered |
| 27 | Last week of the second trimester |
Timings vary with your provider and your pregnancy, so treat this as a rough guide and follow the schedule you're given.
Does the trimester matter for my care?
Not really. Trimesters are a handy way to talk about pregnancy, but your scans, tests and checks are timed by your exact week, not your trimester. That's why it's more useful to know you're "13 weeks 2 days" than "in the second trimester". If your due date has been changed after a scan, your weeks (and trimester) move with it. See can my due date change?.
How Momi helps
Momi works out your week from your last period or conception date and shows your trimester, weeks to go and baby size every day. Each week you'll get tips and articles for that stage, and you can add appointments with medicine reminders. When you reach the third trimester, the kick counter and contraction timer are ready in the same app.
Download Momi free on the App Store or Google Play. Free with ads; optional Premium.
Related: pregnancy trimesters · 13 weeks pregnant · pregnancy week by week
Sources
- ACOG, How Your Fetus Grows During Pregnancy
- ACOG, Committee Opinion 579: Definition of Term Pregnancy
- NHS Best Start in Life, Week 13 (2nd trimester)
- NHS Best Start in Life, Week 28 (3rd trimester)
- Mayo Clinic, 2nd trimester pregnancy
- WHO, Preterm birth fact sheet
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
Is 13 weeks the second trimester?
It depends on who's counting. The NHS week-by-week guide puts week 13 in the second trimester. ACOG and most US sources say the first trimester runs to 13 weeks and 6 days, so the second trimester begins at 14 weeks. Both are correct; they just draw the line in a different place.
Is 12 weeks the end of the first trimester?
In NHS guides, week 12 is the last week of the first trimester. In US sources, the first trimester continues until the end of week 13. Either way, around 12 to 14 weeks is when many people have their dating scan and start to feel less sick.
When does the third trimester start?
At 28 weeks in both NHS and ACOG counting. Week 27 is the last week of the second trimester. The third trimester runs until birth, which for most people is somewhere between 37 and 42 weeks.
How many months is each trimester?
Roughly three months each. The first trimester covers about months 1 to 3, the second about months 4 to 6 and the third about months 7 to 9. Because pregnancy is 40 weeks from your last period, the months are approximate and the third trimester can be longer than three months.
Why are there different trimester dates?
Trimesters are a convenient way of splitting pregnancy into three parts, not strict medical boundaries. Different organizations round the 40 weeks differently. Your care is based on your exact week of pregnancy, not on which trimester you're in.
Related guides
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