How accurate is your due date, really?

The “due date” sounds like an appointment, but it's really the centre of a range. Most pregnancies don't end on that exact day — and that's completely normal. Understanding what the estimate does and doesn't promise takes a lot of the anxiety out of the final weeks.

Only a minority arrive on the date

Birth is a biological process, not a scheduled event. Only a small fraction of babies are born on their estimated due date; most arrive in the couple of weeks on either side of it. A pregnancy is considered full term across a window, not at a single instant — ACOG defines early term as 37 0/7 to 38 6/7 weeks, full term as 39 0/7 to 40 6/7, late term as 41 0/7 to 41 6/7, and post-term from 42 0/7 weeks. The spread of “normal” is deliberately wide.

Why methods disagree by a few days

Each dating method makes different assumptions:

• Last menstrual period assumes you recall the LMP and ovulated on day 14 of a 28-day cycle.
• Ultrasound measures the embryo directly but carries a small measurement range.
• Conception or IVF dating is precise about timing but still estimates the length of the pregnancy.

Because the inputs differ, the outputs can differ by three to four days — sometimes more. That's not error so much as the honest uncertainty in dating a pregnancy. Our calculator shows this by letting you compare methods side by side.

Which estimate should win?

When a reliable first-trimester ultrasound is available, it's generally the most accurate, and guidelines recommend dating by the scan when it differs from the last-period date by more than a small, gestational-age-dependent margin. Early on, a difference of more than about five days is enough to re-date; the allowed difference grows as pregnancy advances. See crown-rump length dating for how that works.

What the due date is good for

Even as an estimate, the due date anchors care: it sets the schedule for screening, defines when a baby is term, and flags when a pregnancy has gone far enough past its date to warrant discussion about induction. It's a planning tool, not a prediction of the exact day.

How to think about it

Treat the due date as the middle of a landing zone that spans a few weeks. Track your gestational age rather than counting down to a single day — the calculator shows exactly where you are today — and let your clinician confirm and adjust the date as needed. If it shifts by a few days after a scan, that's the system working, not a mistake.