Health Pregnancy due date calculator
Estimate a due date from a last period, a conception date or a known due date, with current gestational age, trimesters and key milestones.
Shifts the estimate if your cycle is not 28 days.
Estimated due date
2027-04-08
April 8, 2027
Currently
10w 0d
Trimester 1, 210 days to go
- Conception (est.)
- 2026-07-16
- Weeks elapsed
- 10
- Days remaining
- 210
Milestones
| Milestone | Week | Date |
|---|---|---|
| Second trimester begins | 13 | 2026-10-01 |
| Viability threshold | 24 | 2026-12-17 |
| Third trimester begins | 27 | 2027-01-07 |
| Full term begins | 37 | 2027-03-18 |
| Estimated due date | 40 | 2027-04-08 |
A due date is a midpoint, not a deadline. Only about 4% of babies arrive on the estimated date, and roughly 80% arrive within two weeks either side of it. An early ultrasound dates a pregnancy more accurately than any calculation from a last period, and your midwife or doctor may revise this date.
Ready. Runs locally on your device.
Your files stay on your device. The tool works directly in your browser, using your device to process your files. Nothing is sent to our servers, and we never receive, store, or see your files or figures.
Share result
Cite
Cite this page
Reembun. (2026, July 28). Pregnancy due date calculator. https://reembun.com/pregnancy-due-date-calculator
How to use it
Choose what you know
The first day of your last period, the conception date, or a due date you have already been given.
Enter the date
Then set your average cycle length if it is not 28 days, which shifts the estimate accordingly.
Read the due date and the milestones
The estimate comes with how far along you are today, plus the trimester boundaries, the viability threshold and when full term begins.
Hold it loosely
Only about one birth in twenty happens on the due date. A dating scan is more accurate than any calculation from a date.
Naegele’s rule
The standard calculation, in use since the early 1800s:
due date = first day of last period + 280 daysAdjusted for cycle length, since the rule assumes ovulation on day 14:
due date = LMP + 280 days + (cycle length − 28)280 days is 40 weeks, which is where the familiar number comes from. Note that it counts from the last period rather than from conception, adding about two weeks at the front, so “four weeks pregnant” is roughly two weeks after conception.
Milestones
| Week | Milestone |
|---|---|
| 4 | First missed period; a home test is usually reliable |
| 8 | Embryonic period ends |
| 12 | End of the first trimester; first ultrasound typically offered |
| 13 | Second trimester begins |
| 20 | Anomaly scan usually offered |
| 24 | Conventional threshold of viability |
| 27 | Third trimester begins |
| 37 | Full term begins |
| 40 | Estimated due date |
| 42 | Post-term; induction usually discussed |
What “due date” actually means
The estimate is the median of a distribution, not a target. Published data on spontaneous labour consistently show that only around one birth in twenty-five occurs on the estimated date, and that the spread either side is wide.
| Timing | Approximate share of births |
|---|---|
| Before 37 weeks | 10% |
| 37-38 weeks | 26% |
| 39-40 weeks | 40% |
| 41 weeks | 17% |
| 42 weeks or later | 6% |
Treating a single day as a deadline creates anxiety that the underlying biology does not support. A range of two weeks either side is the more useful mental model.
Why an ultrasound beats the arithmetic
Dating from a last menstrual period assumes you remember the date accurately, that your cycle was typical that month, and that ovulation occurred on schedule. All three are commonly untrue.
A first-trimester ultrasound measures crown-rump length, which varies little between pregnancies at that stage, and dates gestation to within about five days. Most guidelines say that when an early scan and an LMP calculation disagree by more than about a week, the scan wins, and your due date may be formally revised.
What this calculator cannot do
It cannot account for irregular cycles, IVF transfer dates, which have their own dating conventions, or multiple pregnancy, where term is reached earlier. It is arithmetic on the dates you give it.
Anything about your pregnancy belongs with your midwife, obstetrician or doctor. Use this to know roughly where you are in the calendar, not to make decisions.
Common questions
How is a due date calculated?
By Naegele's rule: the first day of the last menstrual period plus 280 days. That assumes a 28-day cycle with ovulation on day 14, so a longer or shorter cycle shifts the date by the difference. This calculator adjusts for cycle length, which many simpler tools do not.
How accurate is a due date?
Only about 4% of babies are born on the estimated date, and roughly 80% arrive within two weeks either side. A due date is the midpoint of a distribution, not a deadline. An early ultrasound dates a pregnancy more accurately than any calculation from a last period.
Why does pregnancy count from before conception?
Because the last menstrual period is a date most people can identify, and conception usually is not. It adds roughly two weeks at the start, which is why you are considered four weeks pregnant about two weeks after conception.
What if my cycle is not 28 days?
Set your actual average cycle length and the estimate shifts accordingly. A 35-day cycle typically means ovulation around day 21 rather than 14, so the due date moves about a week later. Irregular cycles make any LMP-based estimate unreliable, and an ultrasound is the better source.
Last reviewed
