How Accurate Is Due Date? | The Truth About That Calendar

Only about 4-5% of babies are born on their exact estimated due date, and a first-trimester ultrasound is the most accurate way to establish.

That date circled on your calendar feels like a solid promise. You might imagine your baby arriving precisely on that morning, and it’s common to plan leave or announce a timeline around it.

The honest truth is that a due date is an estimate grounded in population averages, not a guarantee. It gives your provider a valuable framework, but your baby is on their own schedule. This article explains how that date is calculated, what makes it shift, and why a five-week window is perfectly normal.

How Providers Calculate That Date

The classic method is Naegele’s rule: count back three months from the first day of your last menstrual period (LMP) and add seven days. It assumes a perfect 28-day cycle with ovulation on day 14. For some people, that math is spot-on.

Many people have cycles that are longer, shorter, or less predictable. If ovulation happened later than day 14, the LMP-based date will be off by days or even weeks. This is the main source of early dating errors.

This is why early ultrasound is so powerful. Measuring the crown-rump length of the embryo gives a more direct read on gestational age that is independent of your cycle quirks. It becomes the gold standard for confirming the timeline.

Why A Single Date Can Feel Misleading

The mismatch between expectation and reality is where the “overdue” anxiety usually starts. Understanding the numbers helps reset expectations around a healthy range rather than a single day.

• The 4-5% Reality: Only about 4-5% of babies arrive on their given due date. It is a specific point in a much broader biological window, not a prediction.
• The Week Of: Only about 35% of births happen in the week surrounding the due date. Most people wait a bit longer or go a bit earlier.
• The Two-Week Window: One analysis of the research suggests roughly 68% of people give birth within 11 days of their estimated date.
• The Five-Week Range: A healthy full-term pregnancy is considered 37 to 42 weeks. This is a normal, healthy range, not a sign that something is wrong if you go past 40 weeks.

Once you see these numbers, the single date feels less like a deadline and more like a guidepost, which is exactly what it is designed to be.

When Ultrasound Refines The Estimate

First-trimester ultrasound, performed up to and including 13 weeks and 6 days, is the single most accurate tool for dating a pregnancy. The margin of error in this window is roughly ±5 to 7 days.

ACOG’s clinical guidance is the standard here. Using the most accurate method for due dates means relying on that early scan if it is available, even if the LMP is known.

After the first trimester, fetal growth patterns become more variable, making second and third-trimester scans less precise for dating. They are excellent for monitoring growth, but not for resetting the clock.

Getting that first scan before 14 weeks is the best way to confirm or correct your EDD early on, which helps keep your prenatal screening windows and delivery planning on track.

When The Dates Don’t Match

A discrepancy between LMP and ultrasound is common, occurring in about 25% of pregnancies. There are clear rules for when providers choose to redate.

1. 7-Day Rule (14 to 15 6/7 weeks): If the ultrasound date differs from LMP by more than 7 days, the EDD shifts to match the ultrasound.
2. 10-Day Rule (16 to 21 6/7 weeks): In the mid-second trimester, a discrepancy of more than 10 days triggers a redate.
3. Unknown LMP: If you cannot recall your LMP or your cycles are highly irregular, dating relies entirely on the earliest ultrasound scan available.
4. Postterm Planning: If the EDD passes and pregnancy reaches 41 to 42 weeks, providers typically discuss induction to reduce postterm risks.

Redating can feel confusing, but it ensures your timeline aligns with the best available evidence for screening tests and delivery decisions.

How Accurate Is A Due Date, Really?

So when people ask how accurate is the estimated due date, the answer depends on the method and timing. A first-trimester scan is quite accurate, with a margin of about ±5 to 7 days.

The NIH resource on this topic confirms that the early scan is the best reference, with research on due date accuracy citing that same ±5 to 7 day margin as the standard for early pregnancy dating.

After the first trimester, fetal growth variability makes scans less precise for determining age. They remain excellent for tracking growth but are not used to set the official clock.

The Bottom Line

Your due date is a medical estimate built from population norms and refined by ultrasound. It is a crucial tool for scheduling tests and managing your pregnancy, but it is not a crystal ball. Expect your baby to arrive in a healthy window, not on a specific date.

If your provider adjusts your EDD after an ultrasound or you feel anxious about going past the date, your obstetrician or midwife can explain how the shift affects your specific screening windows and delivery options.