Clinical Estimated Due Date (EDD) calculator using Naegele's Rule. Enter your LMP with cycle length adjustment for accurate EDD. Also calculates from conception date. Shows gestational age in weeks and days from any past or future date.
Enter the first day of your last menstrual period. Adjust the cycle length if your cycle is not exactly 28 days — this shifts your ovulation date and therefore your EDD. A 30-day cycle adds 2 days; a 26-day cycle subtracts 2 days from the standard EDD.
Your estimated due date is shown using Naegele's Rule (LMP + 280 days, adjusted for cycle length). The formula is: LMP + 1 year − 3 months + 7 days + (cycle length − 28) days. All four trimester boundary dates are listed.
Enter any past or future date to see what gestational age (weeks and days) the pregnancy would be on that date. Useful for planning appointments, understanding milestone timing, or checking gestational age at a past ultrasound date.
Naegele's Rule is the standard clinical formula for calculating estimated due date, developed by German obstetrician Franz Naegele in the 19th century. The formula: Take the first day of the last menstrual period, add 1 year, subtract 3 months, add 7 days. Equivalent to adding 280 days (40 weeks). Example: LMP = 1 July 2024 → +1 year = 1 July 2025 → −3 months = 1 April 2025 → +7 days = 8 April 2025 (EDD). This assumes a 28-day cycle and ovulation on day 14.
280 days (40 weeks) is used because it counts from the first day of the last menstrual period, which typically precedes conception by about 2 weeks. Actual fetal development takes about 266 days (38 weeks) from conception. The 280-day standard was established from large population studies of birth timing and has been validated as the best single estimate for average pregnancy duration in clinical practice.
Gestational age is the age of the pregnancy measured in weeks and days from the first day of the LMP. It is the universal standard in obstetrics because LMP is a reliably known date. Gestational age at birth determines whether the baby is preterm (<37 weeks), term (37–42 weeks), or post-term (>42 weeks). Early ultrasound (before 14 weeks) measures the crown-rump length (CRL) to confirm or revise the LMP-based gestational age.
Both calculate the EDD, but this Pregnancy Due Date Calculator emphasises the clinical Naegele's Rule formula and includes a gestational age lookup feature — enter any date to see the gestational age on that date. It is designed for users who want to understand the medical formula and check gestational age at specific past or future dates (e.g. "how many weeks was I at my 12-week scan last month?").
Standard LMP dating assumes a 28-day cycle with ovulation on day 14. If your cycle is 32 days, ovulation occurs on day 18 (4 days later), so the EDD should be 4 days later than the standard Naegele calculation. Formula: Adjusted EDD = Standard EDD + (cycle length − 28) days. This calculator applies this correction automatically. ACOG and NICE both recommend cycle-length adjustment when dating from LMP.
ACOG (American College of Obstetricians and Gynecologists) recommends: before 9 weeks, use LMP if reliable cycle. 9–14 weeks: if ultrasound CRL differs by >7 days from LMP, revise to ultrasound. 14–16 weeks: revise if differs by >10 days. 16–22 weeks: revise if >14 days different. After 22 weeks: do not revise based on ultrasound alone. This ensures the due date is as accurate as possible based on available information at each stage.
