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Due date calculator
When is my baby due?
The estimated due date is 40 weeks (280 days) after the first day of your last menstrual period, shifted by a day for each day your average cycle differs from 28. Counting from a known conception date instead, it is 38 weeks (266 days). It is an estimate in the truest sense: only about 1 in 20 babies arrive on the date itself, most arrive somewhere between 37 and 42 weeks, and a dating ultrasound in early pregnancy is more accurate than any calendar method — if your care team has given you a date, use theirs.
Estimate your due date
Pick the first day of your last period to see the estimate. If your cycle is not close to regular, a dating scan is the reliable method.
This is the standard calendar estimate, and a dating ultrasound is more accurate than any calendar method. If your pregnancy care team has given you a different date, use theirs.
How due dates are counted
| Counting from | Convention | Notes |
|---|---|---|
| First day of last period | 40 weeks (280 days) | Assumes a 28-day cycle; shift by a day for each day your cycle differs. |
| Conception date | 38 weeks (266 days) | Useful when the date is actually known — IVF transfers, tracked ovulation. |
| Dating ultrasound | Measures the embryo directly | The most accurate method in early pregnancy; overrides the calendar estimate. |
| When babies arrive | 37–42 weeks for most | Only about 1 in 20 arrive on the estimated date itself. |
Why the due date is a window wearing a date's clothing
The single date on the estimate is the middle of a spread, not a deadline. Arrivals cluster through the fortnight either side of it, which is why maternity care talks about "term" — 37 to 42 weeks — rather than the date itself. Planning around the window (bag packed, car seat fitted, backup carer briefed) beats planning around the day.
Which date wins when the estimates disagree
It is common to end up holding two dates: one from this kind of calendar arithmetic and one from the dating scan. Pregnancy care uses the scan, especially when it was done in the first trimester — it measures the embryo rather than assuming anything about your cycle. This calculator is for the weeks before that scan exists, and for satisfying the "when would that make it?" itch the moment the test turns positive.
Due date questions parents ask
- How accurate is a due date calculated from my last period?
- The calendar method assumes ovulation around day 14 of a 28-day cycle, so it drifts with anything that moves ovulation — irregular cycles, recently stopping contraception, breastfeeding. That is why pregnancy care includes a dating ultrasound, which measures the embryo directly and is the most accurate dating method in early pregnancy. When the scan disagrees with the calendar, the scan date is used.
- What percentage of babies are born on their due date?
- Only about 1 in 20 — roughly 5 per cent. The estimated date is the middle of a distribution, not a prediction: most babies arrive somewhere between 37 and 42 weeks, and first babies arrive slightly later on average than subsequent ones.
- Why does the calculator ask about my cycle length?
- The 280-day convention builds in a 28-day cycle with ovulation around day 14. If your average cycle is, say, 35 days, ovulation typically lands about a week later — so the estimate shifts by the same 7 days. The adjustment only helps if your cycles are reasonably regular; if they are not, a dating scan is the reliable method.
- What do the trimesters actually cover?
- The commonly published split is weeks 1–12 for the first trimester, 13–26 for the second, and 27 until birth for the third. The boundaries are conventions for talking about pregnancy rather than biological switches — nothing changes overnight at week 13.
Where these numbers come from
- Pregnancy, Birth and Baby (Australian Government) — How an estimated due date is calculated, and pregnancy week-by-week guidance.
- NHS — Pregnancy — Week-by-week pregnancy guidance, including how pregnancies are dated.
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Stop doing this maths in your head
Once the baby arrives, the questions change to feeds, sleep and nappies — and BubSync keeps that shared record for everyone caring for your baby, from day one.