Estimated Due Date (EDD) Calculator

About the Estimated Due Date (EDD) Calculator

The Estimated Due Date (EDD) Calculator determines your expected delivery date using Naegele's rule — a standard obstetric dating method — with adjustments for cycle length variation. It supports three dating methods: last menstrual period (LMP), ultrasound gestational age, and known conception date.

Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle with ovulation on day 14. This calculator adjusts for non-standard cycle lengths — a 35-day cycle adds 7 additional days, while a 21-day cycle subtracts 7 days, giving a more accurate estimate. First-trimester ultrasound dating (6–13 weeks) is the most reliable method, accurate to ±5 days.

In reality, only about 5% of babies arrive on their exact due date. The full-term delivery window spans 39w0d to 40w6d (ACOG definition), and most births occur within two weeks of the EDD. It shows a complete pregnancy timeline with milestone dates, trimester tracking, gestational-age visualization, and ACOG term classifications to help explain where the pregnancy sits on the usual timeline.

When This Page Helps

Accurate dating is crucial for prenatal care scheduling, genetic screening timing, growth monitoring, and delivery planning. The EDD determines when to perform first-trimester screening (11–13 weeks), anatomy scan (18–22 weeks), Group B Strep testing (35–37 weeks), and when to discuss induction for post-dates pregnancies. This calculator adjusts for cycle length, provides multiple dating methods, and shows the complete milestone timeline.

How to Use the Inputs

1. Select your dating method: LMP (most common), ultrasound, or known conception date.
2. For LMP: enter the first day of your last menstrual period and your average cycle length.
3. For ultrasound: enter the ultrasound date and the gestational age reported.
4. For conception: enter the known or estimated conception date.
5. Review your estimated due date, gestational age, and trimester.
6. Use the pregnancy timeline table to see upcoming milestones.

Example Calculation

Tips & Best Practices

• First-trimester ultrasound (6–13 weeks) is the most accurate dating method: ±5 days. LMP dating is ±2 weeks.
• If ultrasound and LMP dates differ by >7 days in the first trimester (or >14 days later), ultrasound dating is preferred.
• Only 5% of babies arrive on the exact due date — think of it as the middle of a 4-week window (38–42 weeks).
• Cycle length matters: women with 35-day cycles ovulate ~7 days later than women with 28-day cycles, making the true EDD a week later than Naegele without adjustment.
• ACOG recommends against elective delivery before 39w0d due to increased neonatal respiratory and feeding complications.
• After 41w0d, risks of stillbirth and complications increase — most providers recommend induction between 41–42 weeks.

Naegele's Rule: History and Limitations

Franz Karl Naegele published this dating method in the early 19th century. The formula takes the LMP, subtracts 3 months, and adds 7 days (equivalent to adding 280 days). It assumes a 28-day cycle, ovulation on day 14, and typical embryonic development. The rule works well for women with regular 28-day cycles but can be off by 1–2 weeks for women with irregular or non-28-day cycles.

Ultrasound Dating in Practice

First-trimester ultrasound dating is commonly treated as more reliable than LMP-based dating when the two disagree. Crown-rump length (CRL) measured at 7–10 weeks is the most precise biometric measurement. Dating accuracy decreases as pregnancy advances: ±3–5 days in the first trimester, ±1–2 weeks in the second trimester, and ±2–3 weeks in the third trimester. Many obstetric references recommend an ultrasound in early pregnancy when precise dating matters.

The "Due Month" Concept

Many obstetricians and midwives prefer the concept of a "due month" rather than a single due date to set more realistic expectations. The statistical delivery window (38–42 weeks) spans approximately one month. First-time mothers tend to deliver slightly later (40w5d average), while subsequent pregnancies tend to deliver slightly earlier. This variability is normal and does not indicate a problem unless complicated by other factors.

Frequently Asked Questions

• How accurate is the due date?

  A due date calculated from a first-trimester ultrasound is accurate to ±5 days. LMP-based dating is ±2 weeks. Only 5% of babies are born on the exact EDD. About 80% of babies are born between 38–42 weeks. The EDD is best thought of as the center of a probability window, not a precise prediction.

• Why does cycle length matter?

  Naegele's rule assumes ovulation on day 14 of a 28-day cycle. Women with longer cycles ovulate later (e.g., day 21 in a 35-day cycle), meaning the true EDD is later than the unadjusted calculation. Similarly, shorter cycles mean earlier ovulation and an earlier EDD. This calculator automatically adjusts for your cycle length.

• Which dating method is most reliable?

  First-trimester ultrasound (6–13 weeks) is the gold standard because embryonic growth is very consistent and measurable. Crown-rump length (CRL) measurement of 7–10 weeks is most precise (±3–5 days). Second-trimester ultrasound is accurate to ±1–2 weeks. Known conception date (from IVF or confirmed ovulation) is also very accurate.

• What if my dates don't match?

  ACOG guidelines: If first-trimester ultrasound differs from LMP by >5 days (before 9w) or >7 days (9–13w), use ultrasound dating. If second-trimester ultrasound differs by >10–14 days, consider re-dating. Discordance in the third trimester may indicate growth abnormality rather than dating error — do not re-date in the third trimester.

• What is the "full term" window?

  ACOG defines pregnancy terms as: Early term (37w0d–38w6d), Full term (39w0d–40w6d), Late term (41w0d–41w6d), Post-term (≥42w0d). The older definition of "term = 37+ weeks" was revised in updated guidance because outcomes are significantly better at 39–40 weeks versus 37–38 weeks, particularly for neonatal respiratory maturity.

• When should I be concerned about going past my due date?

  The risk of complications (particularly stillbirth) begins to increase after 41 weeks. Most providers recommend induction by 41w0d–42w0d with increased surveillance (biophysical profiles, non-stress tests) starting at 41 weeks. The ARRIVE trial showed that elective induction at 39 weeks for low-risk pregnancies does not increase cesarean rates and may reduce some complications.