Due Date Explained
Your due date is one of the most anticipated numbers in pregnancy. But what exactly is it, how is it calculated, and how accurate is it? This guide explains everything you need to know.
What Is a Due Date?
Your due date (estimated date of delivery, or EDD) is an estimate of when your baby will be born. It is calculated as 40 weeks from the first day of your last menstrual period (LMP).
The due date is not a deadline. It is a midpoint in a range of normal. Most babies are born between 37 and 42 weeks.
How Is the Due Date Calculated?
Naegele's Rule (LMP Method)
EDD = LMP + 280 days
This is the standard method. It assumes a 28-day cycle with ovulation on day 14.
Cycle-Corrected LMP
EDD = LMP + 280 + (cycle length − 28)
This adjusts for cycles that are not exactly 28 days. For example, a 32-day cycle adds 4 days to the due date.
Conception Date
EDD = Conception date + 266 days
This is more accurate if you know the conception date (for example, from IVF or known intercourse).
IVF Transfer
EDD = Transfer date + 263 days (Day 3) or + 261 days (Day 5)
IVF dating is the most precise method because the exact date of fertilization is known.
Ultrasound Dating
EDD = Ultrasound date + (280 − gestational age at ultrasound in days)
First-trimester ultrasound is the most accurate dating method. ACOG recommends using the ultrasound date if it differs from the LMP date by more than 7 days.
How Accurate Is the Due Date?
Only about 5% of babies are born on their exact due date. Most are born within 2 weeks before or after.
The due date is most accurate when:
- Your cycles are regular
- You know the date of your last period
- A first-trimester ultrasound confirms the LMP-based date
What If the Due Date Changes?
It is not uncommon for the due date to change during pregnancy. This usually happens when:
- A first-trimester ultrasound differs from the LMP-based date by more than 7 days
- The LMP date is uncertain
- Cycles are irregular
If your due date changes, your clinician will explain why. A change of a few days is normal and not a cause for concern.
Term Definitions
| Classification | Weeks |
|---|---|
| Preterm | Before 37 0/7 |
| Early term | 37 0/7 – 38 6/7 |
| Full term | 39 0/7 – 40 6/7 |
| Late term | 41 0/7 – 41 6/7 |
| Postterm | 42 0/7 and beyond |
Source: ACOG Committee Opinion, Definition of Term Pregnancy.
When to Call Your Clinician
Contact your healthcare provider if:
- You are unsure of your due date
- Your due date has changed
- You are past your due date
- You have signs of labor before 37 weeks