Pregnancy Due Date Calculator
Calculate your estimated due date and key pregnancy milestones from your last period.
About Naegele's Rule
Naegele's Rule (1850) calculates estimated due date (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period, adjusting for cycle length. Actual delivery occurs within ±2 weeks of EDD in ~80% of pregnancies.
Accuracy Naegele's Rule predicts the correct delivery week in about 80% of pregnancies. An early first-trimester ultrasound (before 14 weeks) is the gold standard for dating and should be used to confirm or adjust the EDD provided here.
Pregnancy Timeline
| Milestone | Week |
|---|---|
| Estimated Conception | ~Week 2 |
| End of 1st Trimester | Week 13 |
| End of 2nd Trimester | Week 26 |
| Full Term | Week 37–40 |
| Estimated Due Date | Week 40 |
What to do with your result
- Book your first prenatal appointment immediately — aim to be seen before 10 weeks to start prenatal vitamins, blood tests, and early ultrasound dating.
- Begin taking folic acid (400–800 mcg/day) if you have not already — it is most critical in the first 12 weeks for neural tube development.
- Mark your trimester end dates on a calendar and research the key screenings scheduled for each trimester (e.g., nuchal translucency scan at 11–13 weeks, anatomy scan at 18–20 weeks).
- Share your estimated due date with your employer early to begin planning maternity leave, and with close family to arrange support for your final weeks.
Frequently Asked Questions
How accurate is Naegele's Rule for calculating a due date?
Naegele's Rule predicts a due date within ±2 weeks of actual delivery in approximately 80% of pregnancies. It is less accurate for women with irregular cycles, those who ovulate late, or those who conceived via IVF (where the fertilization date is known). An early ultrasound (before 14 weeks) provides the most accurate dating.
What should I do immediately after calculating my due date?
Schedule a first prenatal appointment as soon as possible — ideally by 8–10 weeks of pregnancy. Your doctor or midwife will confirm the due date with an ultrasound, begin prenatal care including blood tests, folic acid supplementation, and screen for early pregnancy risks.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period (LMP) and is the standard clinical measure. Fetal age (embryonic age) is counted from conception — approximately 2 weeks after LMP. So a fetus described as 12 weeks gestational age is about 10 weeks old developmentally.
What does it mean if my cycle length is not 28 days?
The standard Naegele calculation assumes a 28-day cycle. This calculator adjusts the due date by the difference between your actual cycle length and 28 days. For example, a 32-day cycle shifts the due date 4 days later. Cycles shorter than 21 or longer than 35 days may indicate irregular ovulation and should be discussed with a healthcare provider.
Is the due date an exact delivery date?
No. The estimated due date (EDD) is simply the midpoint of a normal delivery window. Only about 5% of babies are born exactly on their due date. Births between 37 and 42 weeks of gestation are considered full-term. Your care team will monitor progress and intervene if pregnancy goes significantly past the due date.
This calculator is for educational purposes only. Consult a qualified healthcare professional for medical advice.