Pregnancy Due Date Calculator
Calculate your estimated due date (EDD) using Naegele's Rule — add 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your cycle length.
Advertisement
Calculator
See your Pregnancy Due Date Calculator results
Enter your email to unlock results — free forever.
No spam, ever. Unsubscribe at any time.
Advertisement
Formula
EDD = LMP + 280 days (adjusted for cycle length)
Naegele's Rule assumes a 28-day cycle and 40-week (280-day) gestation. For cycles longer than 28 days, ovulation occurs later, extending the pregnancy duration by the difference. Add the excess days (cycle length − 28) to 280 for your adjusted gestation period.
How to use the Pregnancy Due Date Calculator
- 1
Enter your first day of last period (day of year)
Day 1 = Jan 1, Day 365 = Dec 31
- 2
Enter your average cycle length
Value should be in days.
- 3
Read your results instantly
Results update in real time as you type.
Advertisement
How due dates are calculated
Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1806, is still the primary method used by healthcare providers today. The rule adds 40 weeks (280 days) to the first day of the last menstrual period (LMP), then subtracts 3 months and adds 7 days.
Pregnancy is counted from the LMP rather than conception because LMP is known with more certainty. Actual conception typically occurs 14 days after LMP in a 28-day cycle.
How accurate is the estimated due date?
About 80% of babies are born within 2 weeks of the EDD, and only about 5% are born on the exact date. First-time mothers tend to deliver slightly later than the EDD on average; subsequent births may arrive closer to or before the EDD.
After 8–12 weeks, ultrasound dating using fetal measurements is often more accurate than LMP-based calculation, especially for women with irregular cycles. Healthcare providers may update the EDD after early ultrasound.
Advertisement
The 40-week pregnancy timeline
Pregnancy is divided into three trimesters: First trimester (weeks 1–12) — major organ development occurs; miscarriage risk is highest. Second trimester (weeks 13–27) — often the most comfortable period; fetal movement begins around weeks 18–22. Third trimester (weeks 28–40) — rapid fetal growth; preparation for birth.
Full term is generally considered 39–40 weeks. 'Early term' is 37–38 weeks; 'late term' is 41–42 weeks.
Tips & Insights
Update your due date after first ultrasound
An 8–12 week ultrasound measuring crown-rump length is highly accurate (±5 days) and often more reliable than LMP dating, especially with irregular cycles.
Post-term monitoring
After 41 weeks, most providers recommend increased monitoring. After 42 weeks, induction is typically recommended due to increased risk. Don't be alarmed — 'late' is common and manageable with good prenatal care.
Worked Examples
Standard 28-day cycle
Gestation period: 280 days (40 weeks). Estimated due date: approximately November 20th (day 45 + 280 = day 325 of the year).
Advertisement
Frequently Asked Questions
How accurate is the due date calculation?
Only about 5% of babies are born on their exact due date. Most are born within 2 weeks before or after the EDD. Ultrasound dating is often more accurate than LMP calculation.
What if my cycle is irregular?
Irregular cycles make LMP-based dating less reliable. An early ultrasound (8-10 weeks) is the best method for establishing an accurate due date when cycles are irregular.
What is considered full term?
Full term is 39–40 weeks. Babies born at 37–38 weeks are 'early term.' Before 37 weeks is preterm. After 41 weeks is late term, and after 42 weeks is post-term.
Advertisement