Due Date Calculator
Estimate your due date, trimester timeline, and conception window from your last menstrual period.
Trimester Dates
| Trimester | Start Date | End Date |
|---|---|---|
| First Trimester | -- | -- |
| Second Trimester | -- | -- |
| Third Trimester | -- | -- |
What is Due Date Calculator?
A due date calculator estimates when your baby is likely to arrive based on the first day of your last menstrual period (LMP) and your average cycle length. It uses the same starting point your doctor or midwife uses at an early prenatal visit, so the date this pregnancy due date calculator gives you should closely match the first estimate your provider quotes.
The calculator also shows your likely conception window, current fetal age, and each trimester's date range. Track your gestational age week by week with our pregnancy calculator.
How to Use
- Enter the first day of your last menstrual period.
- Enter your average cycle length in days (default is 28).
- Click Calculate Due Date to see your estimated due date and conception window.
- Review the trimester dates table to understand the timeline of your pregnancy.
Know when you conceived but not your LMP? Use the conception-date method below or our conception calculator; the ovulation calculator pinpoints fertile days.
How the Estimated Due Date Is Calculated
Naegele's rule (LMP method)
The standard obstetric formula, known as Naegele's rule, dates a pregnancy from the first day of the last menstrual period:
Due Date = first day of LMP + 1 year − 3 months + 7 days
That calendar shortcut is equivalent to simply adding 280 days — exactly 40 weeks — to the LMP:
Due Date = LMP + 280 days
- LMP — the first day of your last menstrual period (the day bleeding started, not the day it ended).
- 280 days / 40 weeks — the assumed length of pregnancy counted from the LMP, also called gestational age.
- +1 year − 3 months + 7 days — the mental-math version of the same 280-day count.
Conception-date method
If you know when conception occurred — from IVF, an ovulation test, or cycle tracking — count forward from that date instead:
Due Date = conception date + 266 days
266 days is 38 weeks — the average time from fertilization to birth — and the two methods agree because conception in a typical 28-day cycle occurs about 14 days after the LMP (280 − 14 = 266).
Adjusting for cycle length
Naegele's rule assumes ovulation on day 14 of a 28-day cycle; longer cycles ovulate later and shift the due date later, shorter cycles earlier. This calculator corrects for that:
Adjusted Due Date = LMP + 280 + (cycle length − 28)
For example, a 35-day cycle adds 7 days to the estimate, while a 24-day cycle subtracts 4 days.
Worked Example
Suppose the first day of your last menstrual period was January 10, 2026, with an average 28-day cycle:
- Naegele's rule: January 10, 2026 + 1 year = January 10, 2027; − 3 months = October 10, 2026; + 7 days = October 17, 2026.
- Day-count check: January 10, 2026 + 280 days = October 17, 2026.
- Estimated conception: about 14 days after the LMP, around January 24, 2026, with a fertile window of roughly January 22–26, 2026.
- Conception-method cross-check: January 24, 2026 + 266 days = October 17, 2026.
- Trimester dates shown by the calculator: first trimester January 10 – April 11, 2026; second trimester April 12 – July 18, 2026; third trimester July 19 – October 17, 2026.
With a 35-day cycle the same LMP gives October 24, 2026; with a 24-day cycle, October 13, 2026.
Due Date Reference Table
For a 28-day cycle, each due date below is exactly 280 days after the first day of the last period:
| First Day of Last Period | Estimated Due Date |
|---|---|
| January 1, 2026 | October 8, 2026 |
| February 1, 2026 | November 8, 2026 |
| March 1, 2026 | December 6, 2026 |
| April 1, 2026 | January 6, 2027 |
| May 1, 2026 | February 5, 2027 |
| June 1, 2026 | March 8, 2027 |
Trimester Breakdown and Key Milestones
Standard week ranges and what typically happens in each stage:
| Trimester | Weeks | Key Milestones |
|---|---|---|
| First | 1–13 | Conception and implantation; major organs begin forming; heartbeat usually detectable around week 6; dating ultrasound between weeks 8 and 12. |
| Second | 14–27 | Early nausea often eases; detailed anatomy scan around weeks 18–22; first movements ("quickening") felt around weeks 16–20; viability around week 24. |
| Third | 28–40+ | Rapid fetal weight gain; checkups become more frequent; full term from week 39; the due date falls at exactly 40 weeks. |
The calculator's trimester table splits the 280-day span at days 91 and 190 — the starts of weeks 14 and 28.
How Accurate Is a Due Date?
Ultrasound dating can revise the estimate
An early ultrasound is the most precise way to date a pregnancy: in the first trimester, measuring the embryo's crown–rump length pins the date down to within about 5 to 7 days. If it disagrees with the LMP-based estimate by more than roughly a week, most providers revise the due date to the ultrasound estimate.
Only about 4–5% of babies arrive on their exact due date
A due date is best thought of as the middle of a "due window," not a fixed appointment. Only about 4–5% of babies are born on their exact due date, but the large majority arrive within two weeks either side of it, and a full-term birth is anywhere from 37 to 42 weeks. Plan around the estimate, but expect the real day within the surrounding fortnight.
Frequently Asked Questions
What is Naegele's rule?
Naegele's rule estimates the due date by adding one year, subtracting three months, and adding seven days to the first day of the last menstrual period — equivalent to adding 280 days (40 weeks) and assuming a 28-day cycle.
How does cycle length affect the due date?
Longer cycles usually mean later ovulation, so the due date is pushed back; shorter cycles move it earlier. This calculator adds or subtracts one day for every day your cycle differs from 28.
When is the conception window?
Conception typically happens within a few days around ovulation, roughly 14 days before the next expected period. For a 28-day cycle the calculator shows a 5-day fertile window starting 12 days after the last period began.
What if I know my conception date but not my last period?
Add 266 days (38 weeks) to the conception date. To use this calculator instead, subtract 14 days from the conception date (28-day cycle) and enter that as your last menstrual period.
Can an ultrasound change my due date?
Yes. A first-trimester ultrasound is accurate to within about 5 to 7 days; if it disagrees with the LMP-based date by more than about a week, providers usually revise the due date to the ultrasound estimate.
How accurate is a due date calculator?
Due date estimates are approximate. Only about 4–5% of babies are born on their exact due date, though the large majority arrive within two weeks either side of it. Early ultrasound measurements provide the most accurate dating.
What is the difference between gestational age and fetal age?
Gestational age counts from the first day of the last menstrual period and runs about two weeks ahead of fetal (embryonic) age, which counts from conception. This calculator displays fetal age, so it reads lower than the gestational age your provider quotes.