How to Calculate Your Due Date: Naegele’s Rule & Ultrasound Explained (2026)
Quick Answer
- *Your estimated due date (EDD) is calculated by adding 280 days to the first day of your last menstrual period — this is Naegele’s Rule.
- *A first-trimester ultrasound (7–13 weeks) is accurate to ±5–7 days and is the preferred dating method when it differs from LMP.
- *Only about 4% of babies are born on their exact due date — a normal delivery window spans weeks 37 through 42 (ACOG).
- *“40 weeks pregnant” is measured from your LMP, not conception — the fetus is typically about 38 weeks old at the due date.
What Is a Due Date?
An estimated due date (EDD) is the calendar date at which a pregnancy is expected to reach 40 weeks of gestational age. It’s an estimate — not a deadline. The American College of Obstetricians and Gynecologists (ACOG) reports that only about 4% of babies are born on their exact due date. Most arrive within a two-week window on either side.
Despite the imprecision, having an accurate due date matters. It guides decisions about prenatal testing timing, labor management, and when interventions like induction become appropriate. An off-by-two-week error can mean the difference between a baby considered preterm and one considered full term — with real clinical consequences.
Naegele’s Rule: The Standard Calculation
The most widely used due date formula was published by German obstetrician Franz Karl Naegele in 1812. It has barely changed since. Naegele’s Rule works like this:
EDD = First Day of Last Menstrual Period + 280 days
The practical shortcut: take the first day of your LMP, subtract 3 months, add 7 days, and add 1 year. For example, if your LMP was July 1, 2025, you subtract 3 months (April 1), add 7 days (April 8), and your estimated due date is April 8, 2026.
The formula assumes a 28-day menstrual cycle with ovulation on day 14. That’s not true for everyone. Women with longer cycles (say, 35 days) ovulate later, so a strict LMP calculation will overestimate how far along they are. This is one reason ultrasound dating often takes precedence.
Why 40 Weeks? What “Gestational Age” Actually Measures
Gestational age is counted from the first day of the last menstrual period — not from ovulation or conception. At the moment of conception, you are already considered about 2 weeks pregnant under this system. So when your provider says you are “40 weeks pregnant,” the embryo/fetus is actually about 38 weeks old.
This counting convention exists because clinicians can reliably identify the start of the last period. The exact date of ovulation and fertilization is almost never known with precision. Using the LMP as a universal anchor makes the math consistent and comparable across patients.
How Ultrasound Dating Differs — and When It Wins
Ultrasound dating uses fetal measurements to estimate gestational age independently of the LMP. In the first trimester, the primary measurement is the crown-rump length (CRL) — the distance from the top of the head to the bottom of the spine. There is a well-established relationship between CRL and gestational age during weeks 7 through 13.
According to ACOG Practice Bulletin guidelines, a first-trimester ultrasound is accurate to ±5–7 days. That’s tight enough to be clinically meaningful. A second-trimester ultrasound, by contrast, is only accurate to ±14–21 days because fetal growth becomes more variable between individuals. By the third trimester, ultrasound is a poor dating tool and should not be used to change a previously established due date.
ACOG’s current guidance: if a first-trimester ultrasound differs from the LMP-based EDD by more than 7 days, the due date should be changed to match the ultrasound. If a second-trimester ultrasound differs from the established EDD by more than 14 days, it should be changed. The earlier and more precise the ultrasound, the more it is trusted over the LMP calculation.
Trimester Breakdown
Pregnancy is divided into three trimesters, each covering roughly 13 weeks. Here is what each encompasses:
| Trimester | Gestational Weeks | Key Milestones |
|---|---|---|
| First | Weeks 1–13 | Embryo implantation, organ formation, heartbeat detectable (~week 6), first prenatal visit, nuchal translucency scan (~week 11–13) |
| Second | Weeks 14–27 | Fetal movement felt (~week 18–20), anatomy scan (~week 18–22), gender determination possible, glucose screening (~week 24–28) |
| Third | Weeks 28–40+ | Rapid weight gain, lung maturation, Group B Strep test (~week 36), weekly monitoring begins after week 36 |
Each trimester carries its own risks and screening protocols. The first trimester is when chromosomal abnormalities are most detectable via cell-free DNA testing and nuchal translucency ultrasound. The second trimester anatomy scan checks for structural abnormalities. Third trimester monitoring focuses on fetal growth, position, and placental function.
Preterm, Early Term, Full Term, and Post-Term
Not all pregnancies reaching 37 weeks are equivalent. ACOG updated its terminology in 2013 to better distinguish outcomes across the late-pregnancy spectrum. The definitions matter clinically — babies born at 37 weeks have meaningfully different outcomes than those born at 39 weeks, even though both are technically “term.”
| Category | Gestational Age | Clinical Significance |
|---|---|---|
| Preterm | Before 37 weeks 0 days | Higher risk of NICU admission, respiratory issues, feeding difficulties. According to the CDC (2024), approximately 10.5% of US births are preterm. |
| Early Term | 37 weeks 0 days – 38 weeks 6 days | Increased risk compared to full term; elective delivery before 39 weeks is generally discouraged by ACOG unless medically indicated. |
| Full Term | 39 weeks 0 days – 40 weeks 6 days | Optimal outcomes. ACOG (2017) defines this as the target gestational window for uncomplicated pregnancies. |
| Late Term | 41 weeks 0 days – 41 weeks 6 days | Monitoring intensifies. Induction often discussed. |
| Post-Term | 42 weeks 0 days and beyond | Elevated risk of placental insufficiency, meconium aspiration, and stillbirth. Induction typically recommended. |
What Happens When Your Due Date Changes?
It is common — especially after a first-trimester ultrasound — for a due date to shift by a few days or even a week or two. This can be disorienting, but it reflects the ultrasound providing more accurate information than the LMP estimate.
Once a due date is established using the best available data (usually a first-trimester CRL measurement), it should not be changed based on later ultrasounds. Third-trimester growth scans are not dating tools. They measure fetal size to assess growth, not to recalculate gestational age. A baby measuring “big” or “small” at 32 weeks does not change when the pregnancy started.
If growth scans show the fetus is consistently measuring below the 10th percentile for gestational age, the diagnosis becomes intrauterine growth restriction (IUGR) — a separate clinical issue managed with more frequent monitoring, not a due date revision.
Accuracy: What a Due Date Actually Tells You
Even the best-calculated due date is a probability estimate, not a prediction. Research consistently shows that only about 4% of babies are born on their exact due date. The majority — roughly 70% — are born within 10 days of the EDD in either direction. The EDD marks the center of a normal distribution, not a specific event.
Factors that can influence when labor begins include the baby’s sex (studies suggest male fetuses have slightly longer gestations on average), maternal age, parity (first-time mothers tend to deliver a few days later than women who have delivered before), and genetics. None of these are precise enough to adjust the clinical due date, but they explain some of the natural variation.
For a deeper look at tracking week-by-week milestones and what the pregnancy calculator considers beyond just the due date, see our guide on how pregnancy calculators work.
Find your estimated due date
Use our free Due Date Calculator →Frequently Asked Questions
How is a due date calculated?
Most due dates are calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is Naegele’s Rule. Alternatively, a first-trimester ultrasound can estimate gestational age by measuring the crown-rump length of the embryo, and that measurement is used to set or confirm the due date.
What is Naegele’s Rule?
Naegele’s Rule is a formula first published by German obstetrician Franz Karl Naegele in 1812. To calculate your EDD: take the first day of your last menstrual period, subtract 3 months, add 7 days, and add 1 year. Equivalently, add 280 days to the LMP date. It assumes a 28-day cycle with ovulation on day 14 and remains the standard clinical method worldwide.
How accurate is an ultrasound due date?
Accuracy depends on timing. A first-trimester ultrasound (7–13 weeks) is accurate to within ±5–7 days— the most reliable dating tool available. A second-trimester ultrasound is only accurate to ±14–21 days because individual growth variation increases. According to ACOG, first-trimester dating is preferred whenever it differs from the LMP estimate.
Can my due date change?
Yes. ACOG recommends revising the due date if a first-trimester ultrasound differs from the LMP-based estimate by more than 7 days, or if a second-trimester ultrasound differs by more than 14 days. Once established with first-trimester data, the due date should not be changed based on later growth scans.
What does it mean to be 40 weeks pregnant?
Gestational age is counted from the first day of your last menstrual period, not from conception. At 40 weeks pregnant, the fetus is typically about 38 weeks old from fertilization. ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days. “40 weeks” marks the center of the estimated due date — it is a statistical midpoint, not a deadline.
What happens if I go past my due date?
Going past your due date is common — only about 4% of babies arrive on the exact date (ACOG). Pregnancies at 41 weeks are “late term”; at 42 weeks they are “post-term.” Most providers increase monitoring with nonstress tests or biophysical profiles after 40 weeks and typically recommend induction between 41 and 42 weeks to reduce risks of placental insufficiency and stillbirth.