The due date is one of the first things expectant parents want to know, but it is also one of the most misunderstood numbers in medicine. Only about 4% of babies are born on their exact due date. The due date is better understood as the middle of a window rather than a precise prediction.
How Due Dates Are Calculated
Due Date = LMP + 280 days (40 weeks) LMP = first day of your Last Menstrual Period. This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the estimate shifts accordingly.
Calculation Methods
| Method | Accuracy | Notes |
|---|---|---|
| Last Menstrual Period (LMP) | ±2 weeks | Most common method; assumes regular 28-day cycle |
| First trimester ultrasound | ±5–7 days | Most accurate method; measures crown-rump length |
| Conception date (if known) | ±3–5 days | Add 266 days from conception |
| IVF transfer date | ±1–2 days | Most precise; exact embryo age known |
The Real Delivery Window
Full-term pregnancy ranges from 37 to 42 weeks. About 80% of babies are born between 38 and 42 weeks. First-time mothers tend to deliver slightly later (average 41 weeks and 1 day). The due date is simply the 40-week mark, not a deadline.
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Key Takeaways
- Due date = LMP + 280 days (Naegele's Rule), but only 4% deliver on the exact date.
- First trimester ultrasound is the most accurate dating method.
- Normal delivery range is 37–42 weeks.
- Think of it as a due month rather than a due date.
Frequently Asked Questions
Why is the due date calculated from the last period, not conception?
Because the date of the last menstrual period is usually known with certainty, while the exact date of conception is often unknown. The LMP-based calculation includes the approximately 2 weeks before ovulation and conception actually occurred.
Can the due date change during pregnancy?
Yes. If a first-trimester ultrasound shows the baby measuring more than 7 days different from the LMP-based estimate, most providers will adjust the due date to match the ultrasound measurement.
Is it dangerous to go past the due date?
Most pregnancies that continue to 41 weeks are completely normal. After 42 weeks, risks increase slightly. Healthcare providers typically recommend induction between 41-42 weeks if labor has not started naturally.