Estimate a pregnancy due date using Naegele's Rule. Enter the first day of the last menstrual period to calculate the estimated delivery date.
The Pregnancy Due Date Calculator estimates the expected delivery date (EDD) using Naegele's Rule, the standard method used by healthcare providers worldwide. The rule calculates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP).
Naegele's Rule was developed by German obstetrician Franz Naegele in the early 19th century and remains the primary clinical method for estimating due dates. It assumes a 28-day menstrual cycle with ovulation on day 14. For women with longer or shorter cycles, an adjustment can be made.
The calculator also shows the estimated conception date (approximately 14 days after LMP), the current gestational week if applicable, trimester boundaries, and key milestone dates during pregnancy. While ultrasound dating can be more accurate, the LMP-based calculation provides the initial estimate used in virtually all pregnancies.
Tracking this metric consistently enables professionals to identify patterns in how they allocate time and effort, revealing opportunities to work more effectively and accomplish more each day.
Knowing the estimated due date is essential for prenatal care scheduling, workplace leave planning, and personal preparation. Naegele's Rule provides the standard clinical estimate, and this calculator applies it instantly with additional milestone dates. This quantitative approach replaces vague time estimates with concrete data, enabling professionals to plan realistic schedules and avoid the pattern of chronic overcommitment.
EDD = LMP + 280 days (Naegele's Rule) Cycle adjustment: EDD = LMP + 280 + (cycle_length − 28) days Conception Date ≈ LMP + 14 days (average ovulation) Trimester 1: Weeks 1–12 | Trimester 2: Weeks 13–27 | Trimester 3: Weeks 28–40
Result: Due Date: October 8, 2026
LMP of January 1, 2026 + 280 days = October 8, 2026. Estimated conception around January 15. Trimester 2 starts around March 26. Trimester 3 starts around July 9.
Franz Karl Naegele (1778–1851), a German obstetrician, published this rule based on the assumption that pregnancy lasts 10 lunar months (280 days) from the LMP. Despite being over 200 years old, the rule remains clinically relevant because it closely matches average human gestation.
Irregular cycles, inaccurate LMP recall, and natural variation in gestation length all affect due date accuracy. First pregnancies average 41 weeks + 1 day, while subsequent pregnancies average 40 weeks + 2 days. Ultrasound dating in the first trimester is accurate to about ±5 days.
Workplace leave, hospital pre-registration, nursery preparation, and family arrangements all center around the EDD. Plan for a window of 37–42 weeks rather than a specific date, as healthy full-term deliveries commonly occur anywhere in this range.
Naegele's Rule provides a good estimate, but only about 5% of babies arrive on the calculated date. About 80% are born within 10 days of the EDD. First-time mothers tend to deliver slightly later than the EDD, while subsequent pregnancies may be slightly earlier.
If your cycle is longer than 28 days, add the extra days to the EDD. For a 32-day cycle, add 4 days. For a shorter cycle (e.g., 25 days), subtract 3 days. This adjusts for the likely later or earlier ovulation relative to LMP.
Gestational age is counted from the LMP (the standard 40-week count). Fetal age (conception age) is counted from conception, which is about 2 weeks less. A baby at 40 weeks gestational age is approximately 38 weeks from conception.
Trimester 1: weeks 1–12 (LMP to LMP + 84 days). Trimester 2: weeks 13–27 (LMP + 85 to LMP + 189 days). Trimester 3: weeks 28–40 (LMP + 190 to LMP + 280 days). These are approximate and vary slightly by medical source.
The American College of Obstetricians and Gynecologists defines: early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), and post-term (42+ weeks). Full term is the ideal delivery window.
No. This calculator provides an estimate for informational and planning purposes. Your healthcare provider will determine the official EDD using clinical assessment, LMP, and ultrasound dating. Always follow your provider's guidance for prenatal care.