How Accurate Are Due Dates? The Truth About Your 40-Week Estimate
Only 5% of babies arrive on their due date. Here's how due dates are calculated, why they're often wrong, and what the data actually says about when babies arrive.
The Problem: The 'Due Date' Creates False Expectations
A study in Human Reproduction found that only 4-5% of babies are born on their due date. First-time mothers deliver an average of 8 days past their due date. The 'due date' is really a 'due week.'When Babies Actually Arrive
Step-by-Step: Understanding Your Timeline
Common Mistakes
| When | % of Births | Notes |
|---|---|---|
| 37-38 weeks | 25% | Early term — healthy |
| 39-40 weeks | 40% | Full term — most common |
| 41 weeks | 25% | Late term — normal |
| 42+ weeks | 10% | Post-term — monitoring recommended |
| On due date | 4-5% | Very specific day |
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Frequently Asked Questions
How is the due date calculated?
Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period. Ultrasound dating in the first trimester is most accurate (within 5-7 days). Later ultrasounds can be off by 2-3 weeks.
Can my due date change?
Yes, especially if your early ultrasound differs from your LMP calculation by more than 7 days. Your doctor will use the ultrasound date. After the first trimester, due dates are rarely changed.
What happens if I go past my due date?
Your doctor will monitor you closely after 41 weeks with non-stress tests and amniotic fluid checks. Most doctors recommend induction between 41-42 weeks to reduce the risk of complications.