Quick answer: Pregnancy is dated from the first day of your last menstrual period (LMP). Enter your LMP date above to get how many weeks and days pregnant you are, your trimester, your estimated due date (LMP + 280 days), and your estimated conception date. These are estimates only, confirm them with your healthcare provider.
Last reviewed: October 2026
Key facts
- Due date = LMP + 280 days (40 weeks), the standard Naegele's rule used by doctors.
- Weeks are counted from the LMP, not from conception, so you are already "2 weeks pregnant" at conception.
- Trimesters: first 1-12, second 13-27, third 28+ weeks.
- Fewer than 1 in 25 babies arrives on the exact due date.
A pregnancy calculator tracks your pregnancy from the first day of your last menstrual period. Enter your LMP date, and it shows your current week and day, your trimester, your estimated due date 280 days later, and the date you most likely conceived.
One date, the first day of your last menstrual period, unlocks your entire pregnancy timeline. Doctors use it because it is a date most people can recall, and the 280-day rule (Naegele's rule) built on it matches how pregnancies are dated in medical practice. The one confusing part is that weeks start at the LMP, roughly two weeks before conception actually happens. This free tool lays out your weeks, trimester, due date, and milestones in one view.
Please note: This calculator gives estimates for a typical 28-day cycle. Irregular cycles, long or short cycles, and individual variation shift the dates, and only your healthcare provider or an early ultrasound can confirm your dating.
How to use the pregnancy calculator
- Enter the first day of your last menstrual period using the date picker.
- Click Calculate to see your weeks and days pregnant, trimester, and due date.
- Check the conception estimate and the milestone timeline below it.
How pregnancy dating works (Naegele's rule)
A full-term pregnancy lasts about 280 days from the first day of the last menstrual period. A shortcut doctors use: count back 3 months from the LMP and add 7 days, which gives the same date as adding 280 days. Your current week is simply the days since the LMP divided by 7, shown as weeks and days (for example, 10w3d). Conception most likely happened around 14 days after the LMP in a typical 28-day cycle.
Worked examples
Example 1: LMP of January 1, 2026
Add 280 days: January 1 plus 9 months is October 1, plus 7 days is October 8, 2026. That is the estimated due date. The conception estimate is January 1 + 14 days = January 15, 2026. The first trimester runs from January 1 through March 25, the second through July 8, and the third begins July 9.
Example 2: LMP of June 15, 2026, checked on October 11, 2026
Days from June 15 to October 11 = 118 days. 118 / 7 = 16 full weeks with 6 days remaining, so the result is 16 weeks 6 days. That falls in the 13 to 27 band, so the trimester is the second. The due date is June 15 + 280 days = March 22, 2027.
Pregnancy milestones by week
| Week | Trimester | Milestone |
|---|---|---|
| 4 | First | Missed period; home pregnancy test turns positive |
| 6 - 8 | First | Heartbeat detectable on ultrasound |
| 10 - 13 | First | First-trimester screening tests available |
| 12 | First | End of first trimester; major organs formed |
| 18 - 22 | Second | Anatomy scan; first movements (quickening) |
| 24 | Second | Viability threshold |
| 28 | Third | Third trimester begins; eyes open |
| 37 | Third | Early term: baby considered full-grown enough |
| 40 | Third | Estimated due date |
Trimester breakdown
The first trimester (weeks 1-12) covers conception and organ formation; most pregnancy symptoms like nausea peak here. The second trimester (weeks 13-27) is the growth phase: the anatomy scan happens, movements start, and energy often returns. The third trimester (week 28 onward) is the final stretch of lung maturation and weight gain; full term starts at 37 weeks and the due date lands at 40.
Tips for accurate dating
- Use the very first day of bleeding, not spotting, as your LMP.
- If your cycles run longer or shorter than 28 days, tell your provider: ovulation shifts with cycle length.
- A first-trimester ultrasound is the most accurate dating method; providers may adjust your due date after it.
- Track irregular cycles with an app so your LMP is always at hand.
- Recalculate if your provider updates your due date: work backward 280 days to set the matching LMP.
Common mistakes
- Counting from conception. Pregnancy weeks start at the LMP, about two weeks earlier.
- Expecting the due date exactly. Most babies arrive between 37 and 42 weeks; the date is an estimate.
- Ignoring cycle length. A 35-day cycle pushes ovulation and the real timeline about a week later than the standard formula.
- Using spotting as day 1. Only full flow counts as the cycle start.
Frequently asked questions
How is my due date calculated?
With Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. It assumes a typical 28-day cycle; your provider adjusts it for longer or shorter cycles.
How many weeks pregnant am I?
Count the days since the first day of your last period and divide by 7. For example, 118 days is 16 weeks and 6 days. The calculator above does this for you automatically.
What trimester am I in?
Weeks 1-12 are the first trimester, weeks 13-27 the second, and week 28 onward the third. Enter your LMP above and the calculator shows your current trimester.
When did I conceive?
Most likely around 14 days after your last period started, in a typical 28-day cycle. Subtract about 2 weeks from your gestational age for the fetal age. Our conception calculator estimates the exact date from your due date or LMP.
Why are pregnancy weeks counted from the last period?
Because the LMP is a date most people can remember, while the exact conception day is usually unknown. Ovulation and conception happen around day 14 of a typical cycle, so you are counted as about 2 weeks pregnant on the day you conceive.
Can my due date change?
Yes. If a first-trimester ultrasound gives a different gestational age than the LMP calculation, providers usually adopt the ultrasound dating, which is more accurate early on.
What is my due date if my last period started on 1 March 2026?
Add 280 days to 1 March 2026 to get 6 December 2026. The conception estimate is 14 days after the last period, so 15 March 2026. The third trimester begins at week 28, which for this pregnancy is 6 September 2026 (27 weeks after 1 March).
How many weeks pregnant am I on 1 November 2026 if my last period started on 1 July 2026?
From 1 July to 1 November 2026 is 123 days. 123 / 7 = 17 full weeks with 4 days remaining, so the result is 17 weeks 4 days, in the second trimester. The due date is 1 July 2026 + 280 days = 7 April 2027.
Sources
- ACOG (American College of Obstetricians and Gynecologists): methods for estimating the due date (Committee Opinion 700)
- CDC: pregnancy and prenatal care guidance
Key takeaways
- Due date = first day of your last period + 280 days (Naegele's rule).
- Pregnancy weeks are counted from the LMP, not from conception.
- First trimester: weeks 1-12; second: 13-27; third: 28 onward.
- Conception most likely happened about 14 days after your LMP.
- Estimates only: confirm your dates with your healthcare provider.
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