Pregnancy Calculator · Practical guide
When Does the Second Trimester Start?
Thirteen weeks after your last period the second trimester begins — here is what that means for your dates.
At a glance
The Short Answer
The second trimester begins at 13 weeks 0 days — day 91 counting from the first day of your last menstrual period. It runs through 26 weeks 6 days, and the third trimester takes over at 27 weeks.
Interactive tool
Pregnancy Calculator
Enter your own values, then compare the result with the examples below.
Milestones
| Week | Milestone | Date |
|---|---|---|
| Week 8 | Heartbeat detectable | -- |
| Week 12 | End of first trimester | -- |
| Week 20 | Anatomy scan | -- |
| Week 27 | Start of third trimester | -- |
| Week 40 | Estimated due date | -- |
Trimester Dates for a Real Pregnancy
For an LMP of June 1, 2026, the boundaries fall on these dates:
| Trimester | Gestational Age | Dates |
|---|---|---|
| First | LMP – 12 weeks 6 days | June 1 – August 30, 2026 |
| Second | 13 weeks – 26 weeks 6 days | August 31 – December 6, 2026 |
| Third | 27 weeks – due date | December 7, 2026 – March 8, 2027 |
Why Week 13 Matters
The second trimester is often called the easiest stretch of pregnancy. Morning nausea typically eases, energy returns, and the risk of miscarriage drops sharply. It is also a busy window for screening: the nuchal translucency scan window (weeks 11–14) closes right as it begins, and the detailed anatomy scan happens around weeks 18–22.
Why Cutoffs Do Not Always Match
Some providers quote slightly different trimester boundaries — you may hear week 14 or even the end of week 12 cited as the start. The differences are only convention; this calculator uses clean day counts (first trimester = days 0–90, second = 91–188, third = 189 onward), which is why entering your own LMP above gives you an unambiguous answer.
Frequently Asked Questions
Is week 12 or week 13 the start of the second trimester?
Both conventions exist. This calculator switches at 13 weeks 0 days (day 91 since the LMP), while some providers round the first trimester to 12 or 14 weeks.
When does the third trimester start?
At 27 weeks 0 days from the first day of your last period — day 189. It lasts until delivery, around week 40.
What trimester is 20 weeks?
Week 20 is squarely in the second trimester. It is the halfway point of a 40-week pregnancy and when the anatomy scan is typically scheduled.
How long does the second trimester last?
About 14 weeks — from 13 weeks 0 days through 26 weeks 6 days, which is days 91 through 188 of the pregnancy.