What this calculator does
A due date is conventionally calculated as 280 days, 40 weeks, from the first day of the last menstrual period. This is Naegele’s rule, in use since the early nineteenth century, and it remains the standard starting point in antenatal care.
It assumes a 28-day cycle with ovulation on day 14, so this calculator adjusts for cycle length. Even then it is an estimate: only a small minority of babies arrive on the estimated date, and most births occur across a spread of a couple of weeks either side.
The formula
280 days are added to the first day of the last period, adjusted for any difference between your cycle length and the 28-day assumption. Gestational age is counted from that same starting date, which means the first two weeks precede conception: a convention that surprises many people but is used universally in obstetrics.
| Term | Meaning |
|---|---|
| LMP | Last menstrual period: the first day of bleeding. |
| EDD | Estimated date of delivery. |
| Gestational age | Time elapsed since the LMP, expressed in weeks and days. |
| Term | From 37 weeks 0 days to 41 weeks 6 days. |
The inputs explained
| Field | What to enter |
|---|---|
| First day of last period | The first day of your last menstrual period, before pregnancy. |
| Average cycle length (days) | Your usual cycle length. Longer cycles imply later ovulation and push the due date back accordingly. |
When to use it
Working out the timeline
The trimester dates give the structure of the pregnancy, which is useful for planning appointments, leave and travel.
Checking gestational age today
Antenatal care is scheduled by gestational age in weeks. The figure here tells you where you are in that schedule.
Adjusting for a longer or shorter cycle
A 35-day cycle implies ovulation around a week later than a 28-day one, which moves the due date a week later. Naegele’s rule alone does not make this adjustment; this calculator does.
Comparing against a dating scan
An early ultrasound measures the embryo directly and is generally more accurate than the last period date, particularly with irregular cycles. Where they disagree, clinicians typically use the scan.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How cycle length shifts the due date
The same last period date with different cycle lengths. Dates are generated relative to the day this page was built.
| Cycle length | Estimated due date | Conception (approx.) | Gestational age today |
|---|---|---|---|
| 24 days | 8 Mar 2027 | 15 Jun 2026 | 8 weeks 4 days |
| 26 days | 10 Mar 2027 | 17 Jun 2026 | 8 weeks 4 days |
| 28 days | 12 Mar 2027 | 19 Jun 2026 | 8 weeks 4 days |
| 30 days | 14 Mar 2027 | 21 Jun 2026 | 8 weeks 4 days |
| 32 days | 16 Mar 2027 | 23 Jun 2026 | 8 weeks 4 days |
| 35 days | 19 Mar 2027 | 26 Jun 2026 | 8 weeks 4 days |
Questions
How accurate is a due date?
It is an estimate. Only around one birth in twenty occurs on the estimated date, and most fall within roughly two weeks either side. Its value is in structuring care rather than predicting a day.
Why is pregnancy counted from the last period?
Because it is the last reliably known date. Conception typically occurs about two weeks later, which means the first fortnight of a 40-week pregnancy technically precedes it: a convention that is universal in obstetrics.
What if my cycle is not 28 days?
This calculator adjusts for it. Longer cycles mean later ovulation and a later due date; shorter cycles the reverse. Plain Naegele’s rule does not make this adjustment.
Which is more accurate, this or a dating scan?
An early ultrasound, generally performed between 8 and 14 weeks, measures the embryo directly and is usually more accurate: particularly if cycles are irregular or the last period date is uncertain. Clinicians typically use the scan where the two disagree.
When is a pregnancy considered full term?
From 37 weeks 0 days to 41 weeks 6 days, with 39 to 40 weeks often described as full term specifically. Your care team will explain what applies in your situation.
For cycle tracking before pregnancy, see the ovulation calculator. For elapsed time between any two dates, use days between dates.