Due Date Calculator
Forty Weeks Counted From Before You Were Pregnant
The oddest thing about pregnancy dating is that the clock starts roughly two weeks before conception. Gestational age is counted from the first day of your last menstrual period, not from fertilisation — which means at the moment of conception you are already reckoned "two weeks pregnant". This isn't a quirk; it's practical. The last period is a date most people can name, while the exact day of conception almost never is. Naegele's rule builds on that reliable anchor.
The Formula
This calculator applies Naegele's rule with a cycle-length correction:
Estimated Conception = Last Period Start Date + 14 days + (Cycle Length − 28)
280 days is 40 weeks. The (Cycle Length − 28) term matters more than it looks: the standard rule assumes a 28-day cycle with ovulation on day 14, but ovulation actually tracks the cycle length. On a 35-day cycle you likely ovulated around day 21 — a week later — so the whole pregnancy, and the due date, shifts a week later too.
How Cycle Length Moves the Date
Every row computed from the formula above, using a last period start of January 5, 2026:
| Cycle length | Adjustment | Estimated conception | Estimated due date |
|---|---|---|---|
| 21 days | −7 days | Jan 12, 2026 | Oct 05, 2026 |
| 24 days | −4 days | Jan 15, 2026 | Oct 08, 2026 |
| 26 days | −2 days | Jan 17, 2026 | Oct 10, 2026 |
| 28 days | 0 days | Jan 19, 2026 | Oct 12, 2026 |
| 30 days | +2 days | Jan 21, 2026 | Oct 14, 2026 |
| 32 days | +4 days | Jan 23, 2026 | Oct 16, 2026 |
| 35 days | +7 days | Jan 26, 2026 | Oct 19, 2026 |
| 40 days | +12 days | Jan 31, 2026 | Oct 24, 2026 |
Computed using this calculator's own formula. The spread between a 21-day and a 40-day cycle is 19 days from an identical last period — which is exactly why entering your real average cycle length rather than accepting 28 is worth the effort. Cycle length is accepted between 20 and 45 days.
Trimesters as This Calculator Defines Them
Alongside the due date you get your current gestational age and trimester, based on completed weeks since your last period:
| Completed weeks | Trimester |
|---|---|
| 0 to 12 | 1st Trimester |
| 13 to 26 | 2nd Trimester |
| 27 onward | 3rd Trimester |
Trimester boundaries are conventions rather than biological switches, and different sources draw them a week either way. These are the specific cutoffs this calculator applies.
What a Due Date Actually Predicts
Less than people assume. The due date is the midpoint of a distribution, not an appointment — a single day standing in for a range that spans weeks. Full term is conventionally counted as 37 to 42 weeks, and arriving somewhere in that window is entirely ordinary. Treating the date as a deadline tends to make the final fortnight harder than it needs to be.
Where This Is Genuinely Useful
- An early anchor — before a dating scan, this is usually the only estimate available.
- Planning around the date — leave, cover at work, and travel decisions all need a working figure well before it's confirmed.
- Understanding a scan revision — when an ultrasound moves your date, seeing how cycle length shifts the arithmetic makes the change less mysterious.
- Sense-checking an irregular cycle — if your cycle is nothing like 28 days, the standard rule is quietly wrong for you, and by a predictable amount.
How to Use This Calculator
- Enter the Last Period Start Date — the first day of bleeding, not the last, in YYYY-MM-DD format.
- Enter your Average Cycle Length (days) — first day of one period to the first day of the next. Defaults to 28; accepts 20 to 45.
- Select Calculate for the estimated due date, the arithmetic behind it, an estimated conception date, and how far along you are today with trimester.
If your cycles vary, use a typical average rather than your longest or shortest month.
Related Calculations
To see how far along you are at any point, use the Pregnancy Week Calculator, which works from either a last period date or a known due date. If you conceived via IVF, the IVF Due Date Calculator dates from the embryo transfer instead, which is considerably more precise.