Naegele's Rule adds 280 days (40 weeks) to the last menstrual period date to estimate the due date. It assumes a 28-day cycle and ovulation on day 14. Longer or shorter cycles shift the actual time of ovulation, which is why ultrasound is more accurate for confirming or revising the individual EDD.
Due Date Calculator
Estimated due date by Naegele's Rule — with gestational milestones, current trimester and pregnancy progress.
Naegele's Rule: the formula behind the estimated due date.
Naegele's Rule, used since 1812, adds 280 days (40 weeks) to the last menstrual period date to estimate the due date. It assumes a 28-day cycle and ovulation on day 14 — an average, not a universal rule. In practice, only about 5% of births occur exactly on the calculated due date.
Enter your LMP and see the EDD with full gestational context.
- Enter the date of the first day of your last menstrual period (LMP).
- The calculator shows the estimated due date, current gestational age in weeks and days, trimester and percentage progress.
- The milestone timeline shows the nearest gestational events, already accounting for the current week.
How the EDD is calculated and why it is an estimate
The most common formula to estimate the due date is Naegele's Rule: add 280 days (40 weeks) to the last menstrual period date. This assumes ovulation on day 14 of a 28-day cycle.
Longer or shorter cycles shift the actual time of ovulation and conception. That is why a first-trimester ultrasound — ideally between 8 and 12 weeks — is the most precise method to confirm or revise the estimated due date.
ACOG recommends that the EDD be established by ultrasound when there is a discrepancy of more than 5–7 days between the LMP-based calculation and fetal biometry.
- Full term: 39 weeks to 40 weeks and 6 days.
- Late preterm: 34 to 36 weeks and 6 days.
- Post-term: 42 weeks or more.
What each trimester means clinically
The first trimester (weeks 0–11) is marked by organogenesis: all major organs form. The risk of spontaneous miscarriage is highest during this period.
The second trimester (weeks 12–27) brings rapid growth and first fetal movements. The anatomy scan at week 20 assesses structure, growth and placental position.
The third trimester (weeks 28 until delivery) involves lung maturation, weight gain and preparation for birth. Fetal monitoring intensifies as the due date approaches.
Sources and references for this tool
These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.
- ACOG Committee Opinion — Methods for Estimating the Due DateACOG — Official ACOG reference for EDD estimation, including ultrasound for confirmation.
- Gestational Age — StatPearls (NCBI)NCBI / StatPearls — Clinical basis of Naegele's Rule, ultrasound alternatives and LMP-based calculation limitations.
- WHO — Recommendations on Antenatal CareWorld Health Organization — Global guidelines for antenatal care, including gestational dating and developmental milestones.
- Fetal Development — American Pregnancy AssociationAmerican Pregnancy Association — Detailed fetal development timeline week by week.