The moment you get a due date, it feels wonderfully specific — a single day to count down to. But how much should you actually trust it? Here’s the honest answer.
Is a due date an estimate or a deadline?
It’s an estimate. Only about 1 in 20 babies (roughly 5%) are born on their exact due date. That’s not because the calculation is wrong — it’s because birth naturally happens across a range, and a due date is just the single midpoint of that range.
A healthy, full-term birth can happen anytime between 37 and 42 weeks. So a more realistic way to think about it: your baby will most likely arrive sometime in the two weeks either side of the due date.
A full-term birth can happen anywhere along this band.
When do babies actually arrive?
Births cluster around the due date but spread across several weeks. As a rough picture:
| When | Roughly how many babies |
|---|---|
| Before 37 weeks (preterm) | About 1 in 10 |
| 37–38 weeks (early term) | A growing share |
| 39–40 weeks (full term) | The largest group |
| On the exact due date | Only about 1 in 20 |
| 41–42 weeks (late term) | A smaller share |
The takeaway: arriving in the days or even a couple of weeks around your date is the norm, not the exception.
How is a due date calculated?
The standard method (Naegele’s rule) adds 280 days — 40 weeks — to the first day of your last menstrual period, assuming a 28-day cycle. If your cycle is longer or shorter, the estimate shifts, which is why our Due Date Calculator lets you adjust your cycle length for a more personalised date. From a known conception date, the estimate is about 266 days (38 weeks).
What makes a due date more accurate?
A few things sharpen the estimate:
- An early dating ultrasound (ideally 8–14 weeks) is considered the most accurate way to date a pregnancy. Because all babies grow at a similar rate early on, it can pin your date down to within a few days. If it differs from your last-period estimate, your provider usually goes with the scan.
- Knowing your conception date (for example through ovulation tracking or IVF) removes the guesswork about your cycle.
- Regular cycles make the last-period method more reliable; irregular cycles make it less so.
Can your due date change?
Yes, and it’s common. If an early scan differs from your last-period estimate by more than a few days, your provider will usually re-date the pregnancy from the scan. This isn’t a sign of a problem — it just means the estimate has been refined using better information. Once set, this scan-based date is generally the one your care team sticks with for the rest of the pregnancy, even if later scans suggest your baby is measuring a little ahead or behind.
Why do babies come “early” or “late”?
“Early” and “late” are really just relative to an estimate. First babies, in particular, often arrive a little after the due date. Genetics, your own birth history, and simple natural variation all play a part. Unless your provider has a specific concern, a baby arriving a few days either side of the date is completely normal.
What happens if you go past your due date?
Going a little past 40 weeks is common and usually fine — remember, anything up to 42 weeks is still considered term. As you approach 41–42 weeks, your provider will keep a closer eye on you and your baby, and will usually discuss the option of an induction (helping labour start) if your baby hasn’t arrived. There’s no need to worry the moment your date passes; it simply moves you into a stretch where you’ll be monitored a little more closely.
It’s worth remembering that the “overdue” feeling is often just the due-date illusion at work: if your date had been set a few days later — well within the normal margin of error — you wouldn’t feel late at all. This is exactly why thinking in terms of a window rather than a single day saves so much late-pregnancy anxiety.
Are online due date calculators accurate?
An online calculator is exactly as accurate as the information you give it. Most use the same Naegele’s-rule formula your provider does, so with an accurate last-period date and a typical 28-day cycle, the result is a sound estimate. The two things that improve it are adjusting for your real cycle length (our calculator does this) and, ultimately, an early dating scan, which beats any calculation. Think of a calculator as a great first estimate and a planning tool — not a guarantee of the day itself.
What should you do with your due date?
Use it to plan — when to finish work, when to pack your hospital bag, when to set up the nursery — but hold it loosely. Telling people a range (“sometime in early June”) rather than one exact day can also save you a lot of “any sign yet?” messages in the final stretch.
Want your personalised estimate in a few seconds? Try the Due Date Calculator, then follow along week by week with the Pregnancy Week Tracker.
This article is for general information only and isn’t medical advice. Your healthcare provider is the best source for questions about your due date and pregnancy.