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EDD Calculator

Pregnancy lasts approximately 280 days from the first day of your last menstrual period. This calculator determines your estimated delivery date using two evidence-based methods: Naegele's rule (based on LMP) and ultrasound biometry. Healthcare providers often use both approaches to establish a reliable due date, especially since early ultrasound measurements are accurate to within 3–5 days. The calculator flags discrepancies between methods, which may suggest dating errors or require clinical investigation.

Last updated:

Creators Małgorzata Koperska, MD
1,524 people find this calculator helpful

Understanding the Two EDD Methods

Pregnancy dating relies on two complementary techniques. The LMP method applies Naegele's rule, a straightforward calculation that adds 280 days to the first day of your last menstrual period. This assumes a 28-day cycle and regular ovulation 14 days after menstruation begins. However, this approach becomes unreliable if your cycles are irregular, if you cannot recall your LMP accurately, or if conception occurred in an atypical cycle.

Ultrasound dating uses direct fetal measurements to estimate gestational age. Early scans (first trimester) measure crown-rump length; mid-pregnancy scans assess biparietal diameter, head circumference, femur length, and abdominal circumference. Ultrasound is more accurate earlier in pregnancy—within ±3 days in the first trimester, ±1–2 weeks by the second trimester—because individual fetal growth variation increases as pregnancy progresses.

EDD Calculation Formulas

The two methods apply different mathematical approaches. The LMP formula is straightforward; ultrasound calculation incorporates your cycle length to account for variations in ovulation timing.

EDD (LMP method) = LMP + 280 days

EDD (US method) = Ultrasound date + (281 − (weeks × 7 + days)) + cycle length adjustment

Gestational age = 281 − (EDD − today's date)

  • LMP — First day of your last menstrual period
  • Ultrasound date — Date of your most recent ultrasound examination
  • Gestational weeks and days — Fetal age measured by ultrasound biometry
  • Cycle length — Your typical menstrual cycle duration in days

When EDDs Differ and What It Means

Small discrepancies between LMP and ultrasound EDDs are normal—often 3–5 days—and do not require intervention. Larger gaps warrant investigation, particularly in the first or early second trimester.

If the difference exceeds clinical thresholds, your obstetrician may revise your EDD. The American College of Obstetricians and Gynecologists recommends redating when: before 9 weeks gestation if the difference is more than 5 days; before 16 weeks if the difference exceeds 7 days; before 22 weeks if the difference surpasses 10 days; before 28 weeks if the difference is over 14 days; and at 28 weeks or beyond if the difference is greater than 21 days.

Redating decisions depend on clinical context. Significant early-pregnancy discrepancies may reflect irregular cycles, contraceptive failure timing, or miscalculated LMP. Second or third trimester differences may indicate slower fetal growth (intrauterine growth restriction) or other medical concerns that require investigation.

Important Considerations When Dating Your Pregnancy

Accurate due-date assignment requires careful attention to several factors that affect both calculation methods.

  1. LMP uncertainty limits accuracy — If you are uncertain about your LMP or have irregular periods, LMP-based dating becomes unreliable. Ultrasound measurement is far more trustworthy in these situations. Even a 1–2 week error in LMP can shift your calculated EDD by several days.
  2. Ultrasound timing matters significantly — First-trimester scans provide the most precise gestational-age estimates. Accuracy diminishes progressively: ±3 days in the first trimester, ±1–2 weeks in the second trimester, and ±3–4 weeks in the third trimester. If your first scan occurs late in pregnancy, use LMP as the primary reference if it is reliable.
  3. Cycle length variation affects the US calculation — If your menstrual cycle is longer than 28 days, you ovulate later than the standard assumption, shifting your EDD forward. The calculator adjusts for this; entering your actual cycle length ensures your ultrasound-based EDD reflects your individual physiology.
  4. Discrepancies do not always indicate problems — A 5–7 day difference between methods often reflects normal biological variation and does not signify fetal abnormality. However, persistent or growing differences, particularly if accompanied by unusual ultrasound findings, warrant clinical review to exclude growth concerns or dating errors.

Using Your Results for Pregnancy Planning

Your EDD is an estimate, not a guarantee. Only 5% of babies arrive on their due date. Most pregnancies deliver within a 2-week window around the EDD. Healthcare providers use the EDD to schedule monitoring appointments, assess fetal development milestones, and plan induction or cesarean delivery if medically necessary.

Update your results whenever new information becomes available—for example, if an early ultrasound refines dating. Keep a record of both your LMP and ultrasound EDDs to share with your healthcare team. If your care transfers to another provider, bring your detailed scan report rather than relying on a single EDD figure. Communication with your obstetric team is essential, especially if your two methods diverge or if you have other pregnancy complications.

Frequently Asked Questions

Can I calculate my EDD without an ultrasound?

Yes, if you know your LMP accurately, Naegele's rule provides a reliable estimate. However, ultrasound confirmation is recommended because LMP-only dating has limitations: irregular cycles, contraceptive method failure timing, and recall errors all reduce accuracy. Most clinicians recommend at least one early ultrasound to confirm or revise the LMP-based EDD and rule out multiple pregnancies or structural concerns.

Why does my ultrasound EDD differ from my LMP date?

Discrepancies typically arise because you ovulated later than day 14 of your cycle, leading to a later conception than LMP-based calculation assumes. If your cycle is longer than 28 days, this is expected. Shorter cycles shift your EDD earlier. Small differences (3–5 days) are normal. Larger gaps in early pregnancy may indicate LMP miscalculation or atypical ovulation; later discrepancies may signal growth differences requiring medical evaluation.

Should I worry if my two EDDs are 10 days apart?

It depends on the timing and direction of the difference. In the first trimester, a 10-day gap exceeds the ACOG redating threshold and warrants discussion with your provider. If ultrasound dating is more advanced, you likely ovulated earlier than expected. If ultrasound dating is more delayed, recalculate your LMP or review cycle regularity. In the second trimester or later, a 10-day difference is less surprising due to natural growth variation, but your doctor may investigate if accompanied by other concerns.

How accurate is ultrasound dating in the third trimester?

Third-trimester ultrasound accuracy drops to ±3–4 weeks because individual fetal growth variation increases substantially. If your first reliable ultrasound occurred late in pregnancy, dating based on that scan alone carries significant margin of error. Combining early ultrasound data (if available) with LMP and clinical examination provides the most reliable assessment. Avoid major dating changes late in pregnancy unless there is strong clinical evidence of dating error.

What if my doctor redated my pregnancy?

Redating is common and appropriate when clinical evidence supports a revised EDD. Early redating (first trimester) is reliable and often based on early ultrasound showing a different gestational age than LMP suggested. Later redating is less reliable and usually occurs only if there is significant discrepancy or evidence of growth restriction. Discuss the reason for redating with your provider—it helps ensure appropriate management and delivery timing.

Can I predict my due date based on conception date?

If you know your exact conception date, add 266 days (approximately 38 weeks) to obtain your EDD, since conception typically occurs about 2 weeks after the first day of LMP. However, pinpointing conception is challenging outside of assisted reproduction. For natural conception, LMP or early ultrasound remain the standard because conception timing is often uncertain, particularly in cycles with irregular length or prolonged fertile windows.

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