Calculate your estimated due date using LMP, ultrasound dating, or known conception date. Includes pregnancy timeline, gestational age, and term classifications.
The Estimated Due Date (EDD) Calculator determines your expected delivery date using Naegele's rule — the standard obstetric 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. This calculator provides a complete pregnancy timeline with milestone dates, trimester tracking, gestational age visualization, and ACOG term classifications — everything you need to understand where you are in your pregnancy journey and what comes next.
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.
Naegele's Rule: EDD = LMP + 280 days (+ cycle adjustment) Cycle Adjustment = Actual Cycle Length - 28 days From Conception: EDD = Conception Date + 266 days (38 weeks) From Ultrasound: LMP = Ultrasound Date - GA at Ultrasound; EDD = LMP + 280 days Gestational Age = Days since LMP Trimester 1: 0–12w6d | Trimester 2: 13w0d–26w6d | Trimester 3: 27w0d–delivery
Result: EDD: October 8, 2026
Using Naegele's rule: January 1 + 280 days = October 8, 2026. With a standard 28-day cycle, no adjustment is needed. Conception occurred approximately January 15. First trimester ends around April 2, anatomy scan around May 21, viability (24w) around June 18, and full term begins September 10.
Franz Karl Naegele published this dating method in 1812. 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.
First-trimester ultrasound dating has largely replaced LMP-based dating in developed countries. 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. ACOG recommends at least one ultrasound between 10–13 weeks for optimal dating.
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.
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.
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.
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.
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.
ACOG defines pregnancy terms as: Early term (37w0d–38w6d), Full term (39w0d–40w6d), Late term (41w0d–41w6d), Post-term (≥42w0d). The old definition of "term = 37+ weeks" was replaced in 2013 because outcomes are significantly better at 39–40 weeks vs. 37–38 weeks, particularly for neonatal respiratory maturity.
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.