How Accurate Is Your Due Date?

A due date is presented as a specific day, which makes it feel like a prediction. It is better understood as the centre of a range — and knowing how wide that range is makes the number considerably more useful.

About 1 in 20

Only around 5% of babies are born on their estimated due date. That figure comes from ACOG guidance and is worth sitting with, because almost everything else about how due dates are discussed implies more precision than that.

This is why our Pregnancy Due Date Calculator carries an estimate-only confidence rating — the lowest of any calculator on the site. That is not a limitation of our implementation. It is an honest reflection of what last-menstrual-period dating can tell you, and the confidence meter shows the fewest filled segments precisely so the number is not read as more certain than it is.

Where the uncertainty comes from

Two distinct sources, and they stack:

1. The dating method. Naegele's rule assumes a 28-day cycle with ovulation at day 14 — see how due dates are calculated. Real cycles vary, so the starting point itself carries uncertainty.
2. Normal variation in gestation length. Even with perfect dating, pregnancies do not all last exactly 280 days. Full-term birth spans a range of weeks, not a single day.

The first source can be reduced with better dating. The second cannot be reduced at all — it is simply how birth timing works.

Why an early scan is more accurate

ACOG considers first-trimester ultrasound more accurate than LMP-based calculation for dating a pregnancy. A scan measures the developing pregnancy directly, rather than working backward from a cycle date and an assumed ovulation day.

If your scan date and your calculated date differ, use the scan date. If your maternity team has revised your due date after a scan, that revision is a refinement based on better information, not a sign that anything is wrong.

What the estimate is genuinely good for

Despite the width of the range, the estimate does real work:

• Tracking gestational age. Knowing roughly how many weeks along you are guides appointment scheduling, screening windows, and what to expect — see pregnancy weeks and trimesters.
• Planning around a window. Leave, childcare, and practical arrangements are better built around a span of a couple of weeks than a single date.
• A reference point for your care team. Clinical decisions are made relative to gestational age, so having a working date matters even though it is approximate.

Setting expectations

If it helps, treat the due date as the middle of a window rather than a target. Arriving before or after it is ordinary, not a deviation from a plan.

It is also worth knowing that a due date can legitimately change during pregnancy, usually after an early scan. That is the dating being improved, not the pregnancy behaving unexpectedly.

Anything that concerns you about timing, or about your pregnancy generally, is a question for your midwife or doctor rather than a calculator. They can see your full history; this tool sees one date.

The bottom line

About 1 in 20 babies arrives on the estimated due date. The estimate is useful for tracking gestational age and planning within a window — not for predicting a day. If you have had a first-trimester scan, its date is the more accurate one, and your care team's date is the one to work from.