Pregnancy Due Date Calculator
Estimate a due date from your last menstrual period or conception date, and see current gestational age, trimester and key milestone dates.
Count from the first day of bleeding, not the last.
Naegele's rule assumes a 28-day cycle. A longer or shorter cycle shifts the due date by the difference, this tool applies that correction.
Choose a date and your usual cycle length, then calculate.
Estimated due date
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Nearly every due date given anywhere in the world comes from a formula published in the early 1800s by the German obstetrician Franz Karl Naegele. It survives because it is simple and because, on average, it works: add 280 days to the first day of the last menstrual period.
The rule is usually taught in a form that is easier to do in your head, take the first day of your last period, add one year, subtract three months, add seven days. The result is the same 280 days, give or take the length of the months involved. This calculator does the arithmetic on real calendar dates rather than the mental shortcut, so leap years and month lengths are handled exactly.
Behind that simplicity sits a single assumption that does an enormous amount of work: that ovulation occurred on day 14 of a 28-day cycle. Everything else follows from it. Forty weeks from the period is really thirty-eight weeks from conception plus the fourteen days the rule assumes came before it.
Cycle length varies substantially between people and somewhat between cycles for the same person. What stays comparatively fixed is the luteal phase, the interval from ovulation to the next period, which is typically around fourteen days regardless of total cycle length. So a longer cycle means a later ovulation, not a longer wait afterwards.
That gives a clean correction: for every day your cycle exceeds 28, add a day to the due date. For every day shorter, subtract one.
| Cycle length | Estimated ovulation day | Correction to due date |
|---|---|---|
| 21 days | Day 7 | 7 days earlier |
| 25 days | Day 11 | 3 days earlier |
| 28 days | Day 14 | No change, the textbook case |
| 32 days | Day 18 | 4 days later |
| 35 days | Day 21 | 7 days later |
A fortnight separates the 21-day and 35-day estimates for the same LMP date. This is the single largest source of disagreement between due-date calculators.
The correction has limits. It assumes a regular cycle, and it assumes the luteal phase really is fourteen days for you. If your cycles are irregular the LMP date itself becomes unreliable, and dating by ultrasound is considerably more accurate, which is exactly why an early scan is offered.
Where it works well
- Cycle-length correction applied, so the estimate is not locked to a 28-day assumption
- Accepts either an LMP date or a known conception date
- A full milestone timeline, trimester boundaries, viability, term categories, not just a single date
- Shows how far along you are today in weeks and days, the format clinical notes use
- Nothing is transmitted or stored; the dates never leave your browser
Where it falls short
- Menstrual dating only, it cannot incorporate ultrasound measurements, which are more accurate in the first trimester
- Assumes a regular cycle and a roughly fourteen-day luteal phase
- Not designed for twins or higher multiples, which are typically delivered earlier
- It is arithmetic, not care, it knows nothing about your history and cannot replace a midwife or doctor
Two different numbers describe how far along a pregnancy is, they differ by exactly two weeks, and mixing them up is the most common confusion in the whole subject.
| Gestational age | Fetal (conceptional) age | |
|---|---|---|
| Counted from | First day of the last menstrual period | Estimated date of conception |
| Full term at | 40 weeks | 38 weeks |
| Used by | Clinicians, maternity notes, scan reports, virtually all official guidance | Embryology texts and some app-based week-by-week descriptions |
| Advantage | Based on a date you can actually observe and recall | Reflects actual developmental age |
| Drawback | Includes two weeks before the pregnancy biologically began | Requires knowing the conception date, which most people do not |
This calculator reports gestational age, because that is what your notes, your scans and every appointment will use.
The practical consequence: if an app tells you the baby is "eight weeks developed" and your notes say ten weeks, both are right and they are describing the same pregnancy. When comparing anything against anything, check which clock is being used. There is more on this in gestational age versus fetal age.
"Term" used to mean anything from 37 to 42 weeks, treated as a single safe window. That changed: the category was subdivided because outcomes across that five-week range are not uniform, and a birth at 37 weeks is measurably different from one at 39.
| Category | Gestational age |
|---|---|
| Preterm | Before 37 weeks |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post-term | 42 weeks 0 days and beyond |
These boundaries are the reason elective procedures are generally not scheduled before 39 weeks without a clinical reason. Definitions and thresholds are set by professional bodies and may be updated, your care provider works to the current version.
The timeline in the calculator marks each of these boundaries as a date, so you can see when each threshold is reached rather than having to count weeks forward from a due date.
The most useful thing to understand about a due date is that it is the centre of a distribution rather than a prediction of an event. Very few babies arrive on the estimated date. The great majority arrive somewhere in a window of roughly two weeks either side of it, and that spread is normal biology rather than error in the calculation.
This has practical implications worth planning around. Arrangements pinned to the exact due date, a partner's leave starting that morning, a relative flying in for that week, a car booked for that day, are pinned to the least likely single outcome. Planning for a window from about 37 weeks onwards is the version that survives contact with reality.
It also explains why the difference between due-date calculators matters less than it appears. A four-day disagreement caused by cycle length sits well inside the natural spread of arrival dates. The correction is worth applying because it makes the midpoint more accurate, not because it narrows the window.
To count the weeks between any two dates yourself, use the Date Difference Calculator. Once the baby arrives, the Age Calculator reports age in weeks and months, which is the unit used for developmental milestones and vaccination schedules through the first two years.