A Date That Was Never a Promise
At the 12-week scan, the sonographer writes down a date. From that moment it becomes the date — the one you tell family, the one that goes in the calendar, the one you count down to.
It's worth knowing what that date actually represents statistically, because it isn't a prediction of when your baby will arrive. It's the midpoint of a fairly wide distribution.
Only around 4% of babies are born on their estimated due date. Not because dating is unreliable, but because a due date is one day in a range that spans several weeks, and the range is what's normal.
What the Distribution Looks Like
Among pregnancies that begin labour spontaneously, births cluster around 39–41 weeks with a long tail on either side. Roughly:
- The single most likely week is 39–40 weeks
- The large majority of births fall between 37 and 42 weeks
- A meaningful minority arrive before 37 weeks
- A smaller proportion continue past 42 weeks without intervention
The distribution is not symmetric. It has a longer tail on the early side than the late side, partly because obstetric practice intervenes before pregnancies extend very far past term.
Two factors shift the whole curve noticeably:
First pregnancies tend to run longer. First-time mothers are more likely to deliver after their due date than mothers who have given birth before. The difference is on the order of days rather than weeks, but it's consistent enough that it's worth knowing if this is your first.
Individual variation is real. Analyses of naturally-conceived pregnancies with precisely known conception dates have found substantial variation in pregnancy length between individuals — considerably more than the neat "40 weeks" figure suggests. Some people simply carry longer than others, and that pattern tends to repeat across their pregnancies.
Term Isn't One Category Anymore
Obstetric guidance moved away from treating everything from 37 to 42 weeks as equivalent, because outcomes differ meaningfully across that range. The current terminology in widespread use:
| Category | Gestation |
|---|---|
| Preterm | before 37 weeks |
| Early term | 37 weeks 0 days – 38 weeks 6 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days |
| Post term | 42 weeks 0 days and beyond |
The reason for the split was evidence that babies born in the early term window have somewhat higher rates of respiratory difficulty and other complications compared with those born at full term. That finding directly changed practice: elective deliveries without a medical indication are now generally not recommended before 39 weeks.
If you're offered a scheduled birth, the 39-week threshold is a useful thing to understand, because it's the reasoning behind the timing.
Calculating Your Date
The Due Date Calculator works from the standard convention:
- Enter the first day of your last menstrual period.
- Adjust the cycle length if yours differs from 28 days.
- Read the estimated due date and the current gestational age.
The underlying method adds 280 days to the LMP date. That assumes a 28-day cycle with ovulation on day 14, which is why cycle length adjustment matters — someone with a consistent 35-day cycle typically ovulates later, which shifts the estimate by around a week.
Early ultrasound dating is more accurate than LMP dating for most people, because it measures the embryo directly rather than inferring from cycle timing. A first-trimester crown-rump length measurement is the most precise dating available, with accuracy narrowing considerably compared to later scans. If an early scan disagrees with your LMP date by more than a few days, clinical guidance generally favours the scan.
Dating accuracy decreases as pregnancy progresses, because babies grow at increasingly variable rates. A scan at 30 weeks estimates size well and gestational age poorly.
Why the Date Still Matters
Given the width of the distribution, it's fair to ask what a due date is for. Several things depend on it:
Screening windows. Combined first-trimester screening has a specific gestational window. The anomaly scan is typically scheduled in a defined range. Gestational diabetes screening, and in some regions group B strep testing, have their own timing. Miss the window and the test either can't be done or becomes less informative.
Intervention thresholds. Decisions about when to offer induction depend on gestational age, and those thresholds are defined in weeks and days from the due date.
Interpretation of measurements. Whether a baby's growth is on track is assessed against gestational age. An inaccurate date makes growth assessment less meaningful.
Practical planning. Leave arrangements, childcare for older children, travel restrictions imposed by airlines, and hospital bag timing all key off the date.
So the date is genuinely useful — as a reference point for clinical timing rather than as a forecast of arrival.
Planning for a Range, Not a Day
The most useful mental shift is to plan around a window.
Be ready from 37 weeks. Bag packed, hospital route known, arrangements for other children or pets in place, work handover documented. A meaningful proportion of babies arrive before 39 weeks.
Expect to still be pregnant on your due date. Particularly with a first baby. The psychological difference between "I'm overdue" and "this is entirely normal" is significant, and the second framing is the accurate one.
Tell people a window rather than a date. "Sometime in the second half of March" prevents the flood of check-in messages that starts the morning after a stated due date passes.
Understand your local induction policy in advance. Practice varies between countries and between hospitals on when induction is offered for a pregnancy continuing past term. Knowing the local approach — and what the options are — before you're at 41 weeks makes the conversation easier.
Plan leave with a buffer. Starting leave at 38 weeks and delivering at 41 means three weeks of leave spent waiting. Starting at 40 and delivering at 37 means no preparation time at all. Where you have flexibility, think about which error you'd rather make.
Common Misunderstandings
"Overdue" starting the day after the due date. Clinically, pregnancy isn't considered post-term until 42 weeks. The days between 40 and 42 weeks are a normal part of the distribution.
Assuming a late baby means something is wrong. Continuing past the due date is common and, within the monitored window, expected. Monitoring increases past term precisely because that's how risk is managed.
Treating a late-pregnancy scan estimate of fetal weight as precise. Ultrasound weight estimates carry a substantial margin of error, and that error is well documented. A late scan suggesting a large baby is an estimate, not a measurement.
Expecting the due date to be revised repeatedly. Once a due date is established from an early scan, it's generally not changed by later scans, because later dating is less accurate. A later scan showing a baby measuring ahead or behind is information about growth, not about dates.
FAQ
What percentage of babies are born on their due date? Around 4%. The date marks the middle of a distribution rather than a likely single day.
Do first babies really come later? On average, yes — first-time mothers are more likely to deliver after their due date than those who have given birth before. The difference is measured in days.
Which is more accurate, LMP or ultrasound dating? First-trimester ultrasound, particularly crown-rump length measurement. It's the preferred method where the two disagree significantly.
Why does cycle length change my due date? The standard calculation assumes ovulation on day 14. A longer cycle usually means later ovulation, which shifts conception and therefore the estimate.
What's the difference between early term and full term? Early term is 37–38 weeks and 6 days; full term is 39 weeks to 40 weeks and 6 days. The distinction exists because outcomes differ modestly between the two windows.
When would induction typically be offered? This varies by country, hospital, and individual circumstances. It's a conversation to have with your own midwife or obstetrician, who can explain the local approach and what applies to your pregnancy.
The Takeaway
A due date is a well-calculated reference point sitting in the middle of a several-week distribution. Treating it as a window rather than a deadline makes the last month considerably less stressful — and makes it easier to plan for a baby who might reasonably arrive any time from 37 weeks onward.
Calculate your estimated due date free with the Due Date Calculator at sadiqbd.com — no sign-up, instant results. This article is general information; for guidance on your own pregnancy, speak with your midwife, obstetrician, or healthcare provider.
