The Quick Answer
A pregnancy due date is an estimated date of delivery (EDD), most commonly calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP).
For example, if the LMP was January 1, the estimated due date is October 8 (280 days later).
Only about 4-5% of babies are born on their exact due date. The EDD is the midpoint of a normal delivery window, not a precise prediction. According to the American College of Obstetricians and Gynecologists (ACOG), the most accurate dating uses first-trimester ultrasound, which is precise to within 5-7 days.
Naegele's Rule
Naegele's rule is the standard method taught in medical education and used in clinical practice. Named after German obstetrician Franz Naegele (1778-1851), the rule is:
EDD = LMP + 280 days
A common shortcut formula for mental calculation:
EDD = LMP date - 3 months + 7 days + 1 year
This shortcut works because subtracting 3 months and adding 7 days is approximately equivalent to adding 280 days. The small discrepancy (1-2 days) arises from the varying number of days in each month.
Assumptions built into Naegele's rule
- The menstrual cycle is 28 days long
- Ovulation occurs on day 14 of the cycle
- Fertilization occurs on or near the day of ovulation
- The first two weeks of "pregnancy" predate actual conception
These assumptions mean Naegele's rule is most accurate for women with regular 28-day cycles.
Worked Example 1: LMP = March 15, 2026
Using the 280-day method:
March 15, 2026 + 280 days:
- March 15 to March 31 = 16 days
- April = 30 days (running total: 46)
- May = 31 days (77)
- June = 30 days (107)
- July = 31 days (138)
- August = 31 days (169)
- September = 30 days (199)
- October = 31 days (230)
- November = 30 days (260)
- December 1-20 = 20 days (280)
EDD = December 20, 2026
Using the shortcut:
- March 15 - 3 months = December 15
- December 15 + 7 days = December 22
- Same year (since December is still in the forward direction): December 22, 2026
The two methods differ by 2 days. This is because March has 31 days while the shortcut treats all months as equivalent. The 280-day count is the precise method; the shortcut is an approximation for quick mental math.
Worked Example 2: LMP = July 1, 2026
Using the 280-day method:
July 1, 2026 + 280 days:
- July 1 to July 31 = 30 days
- August = 31 days (61)
- September = 30 days (91)
- October = 31 days (122)
- November = 30 days (152)
- December = 31 days (183)
- January = 31 days (214)
- February = 28 days (242)
- March = 31 days (273)
- April 1-7 = 7 days (280)
EDD = April 7, 2027
Using the shortcut:
- July 1 - 3 months = April 1
- April 1 + 7 days = April 8
- Add 1 year: April 8, 2027
Again, a 1-day difference due to month-length variation. Both are clinically acceptable -- the EDD is an estimate, not a guarantee.
Conception-Based Calculation
If the conception date is known (or closely estimated), the calculation is simpler:
EDD = conception date + 266 days (38 weeks)
In a standard 28-day cycle, conception occurs approximately 14 days after the first day of the LMP. This means:
- LMP + 280 days = conception + 266 days (the same date, approached from two starting points)
Example: If conception occurred on March 29, 2026:
March 29 + 266 days = December 20, 2026
This matches the LMP-based calculation from Worked Example 1 (LMP March 15 + 14 days = ovulation/conception around March 29).
Conception-based dating is particularly useful for IVF pregnancies, where the fertilization date is known precisely.
Ultrasound Dating
Ultrasound measurement is the most accurate method for establishing a due date, particularly in the first trimester.
How it works
In early pregnancy (6-13 weeks), ultrasound measures the crown-rump length (CRL) -- the distance from the top of the embryo's head to its rump. Early embryonic growth follows a highly predictable pattern across virtually all pregnancies, making CRL a reliable indicator of gestational age.
In the second and third trimesters, ultrasound uses multiple measurements including biparietal diameter (head width), femur length, and abdominal circumference. However, individual variation in fetal size increases as pregnancy progresses, reducing dating accuracy.
When ultrasound overrides LMP
According to ACOG Committee Opinion 700, the ultrasound-based EDD should replace the LMP-based EDD when the discrepancy exceeds:
- More than 5 days in the first trimester (up to 8 weeks 6 days)
- More than 7 days at 9 weeks 0 days through 15 weeks 6 days
- More than 10 days at 16 weeks 0 days through 21 weeks 6 days
- More than 14 days at 22 weeks 0 days through 27 weeks 6 days
Beyond 28 weeks, ultrasound is generally not used to change the due date.
Accuracy Comparison
| Method | Typical accuracy | When used | Notes |
|---|---|---|---|
| Naegele's rule (LMP) | +/- 14 days | First prenatal visit | Assumes 28-day cycle; free and immediate |
| First-trimester ultrasound | +/- 5-7 days | 6-13 weeks | Most accurate single method |
| Second-trimester ultrasound | +/- 10-14 days | 14-27 weeks | Less precise due to size variation |
| Third-trimester ultrasound | +/- 21-28 days | 28+ weeks | Too imprecise for dating |
| Conception-based | +/- 3-5 days | When conception date is known (IVF) | Most precise when date is certain |
The best clinical practice, as recommended by ACOG and the National Institute of Child Health and Human Development (NICHD), combines LMP dating with first-trimester ultrasound confirmation.
Cycle Length Adjustment
Naegele's rule assumes a 28-day cycle. For women with consistently longer or shorter cycles, adjust the EDD:
Adjusted EDD = standard EDD + (actual cycle length - 28)
Example: A woman with a 35-day cycle and LMP of March 15, 2026:
- Standard EDD = December 20, 2026
- Adjustment = 35 - 28 = +7 days
- Adjusted EDD = December 27, 2026
The logic: in a longer cycle, ovulation occurs later (approximately day 21 instead of day 14), so conception occurs later, and delivery is expected later by the same number of days.
For irregular cycles, first-trimester ultrasound is strongly preferred over any LMP-based calculation.
Full-Term Definitions
ACOG defines the following categories based on gestational age:
| Term | Gestational age |
|---|---|
| Preterm | Before 37 weeks 0 days |
| Early term | 37 weeks 0 days - 38 weeks 6 days |
| Full term | 39 weeks 0 days - 40 weeks 6 days |
| Late term | 41 weeks 0 days - 41 weeks 6 days |
| Post-term | 42 weeks 0 days and beyond |
The due date (40 weeks 0 days) falls squarely within the full-term window. The range from 37 to 42 weeks encompasses when approximately 80% of deliveries occur.
Why Only 4-5% of Babies Arrive on the Due Date
The EDD is a statistical midpoint, not a prediction. Human gestation length naturally varies due to:
- Genetic factors -- gestational length runs in families
- First pregnancies -- first-time mothers tend to deliver slightly later on average
- Maternal age -- older mothers tend toward slightly longer gestations
- Fetal sex -- some studies suggest male fetuses are carried slightly longer
- Uncertainty in dating -- even with ultrasound, there is inherent measurement variability
A 2013 study published in Human Reproduction found that natural variation in gestation length spanned about 37 days (5 weeks) even after excluding preterm births and accounting for accurate dating.
Frequently Asked Questions
How accurate is a pregnancy due date?
About 4-5% of babies arrive on the exact EDD. Most are born within a two-week window on either side. First-trimester ultrasound narrows the estimate to +/- 5-7 days; LMP-based dating is accurate to about +/- 14 days.
Can my due date change during pregnancy?
Yes. If a first-trimester ultrasound disagrees with the LMP-based date by more than 5-7 days, ACOG recommends updating the EDD to match the ultrasound. Later ultrasounds are less likely to change the date because their margin of error is larger.
What if my cycle is irregular?
Naegele's rule assumes a 28-day cycle and becomes less reliable with irregular cycles. First-trimester ultrasound is the preferred dating method for women who do not have regular cycles.
Is the due date 40 weeks from conception or from LMP?
From LMP. The 40-week (280-day) count starts from the first day of the last menstrual period. Since conception occurs roughly 2 weeks later, the actual development time is about 38 weeks (266 days) from conception.
What percentage of babies are born on their due date?
Approximately 4-5%. About 80% of babies are born between 37 and 42 weeks of gestation.
Why is pregnancy counted from the last period and not conception?
Because the LMP date is reliably known by most women, while the exact conception date is usually unknown. This convention has been standard since Naegele formalized it in the early 1800s.
How does ultrasound determine the due date?
First-trimester ultrasound measures crown-rump length (CRL), which correlates closely with gestational age because early embryonic growth is highly uniform. Later ultrasounds measure head size, femur length, and abdominal circumference, but individual variation makes them less precise for dating.
What is the difference between gestational age and fetal age?
Gestational age counts from the LMP (40 weeks at term). Fetal age counts from conception (38 weeks at term). Gestational age is the clinical standard. Fetal age is approximately 2 weeks less.
Does the due date change for IVF pregnancies?
IVF due dates are calculated from the embryo transfer date, which is more precise than LMP. For a day-5 blastocyst transfer, the EDD equals the transfer date plus 261 days.
What does full term mean?
ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37-38 weeks, late term is 41 weeks, and post-term is 42 weeks and beyond.
Calculate Your Due Date
Use the due date calculator to estimate your EDD based on your last menstrual period or conception date. For related tools, try the conception calculator or the date difference calculator.
This content is for educational purposes only and does not constitute medical advice. Prenatal care should be managed by a qualified healthcare provider.