Pregnancy Due Date Calculator

About the Pregnancy Due Date Calculator

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.

When This Page Helps

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 together with the related milestone dates.

How to Use the Inputs

1. Enter the first day of the last menstrual period (LMP): year, month, day.
2. Optionally adjust the cycle length if it differs from 28 days.
3. View the estimated due date (EDD).
4. See the estimated conception date and trimester boundaries.
5. Use the trimester dates for prenatal care scheduling.

Example Calculation

Tips & Best Practices

• Naegele's Rule assumes a 28-day cycle; adjust for longer/shorter cycles.
• Only about 5% of babies are born on their exact due date.
• Most births occur within 2 weeks of the due date (38–42 weeks gestation).
• Early ultrasound (8–12 weeks) can refine the EDD if it differs from the LMP calculation.
• Full term is 39–40 weeks; early term is 37–38 weeks; late term is 41 weeks.
• This calculator is for educational purposes—consult your healthcare provider for clinical guidance.

Naegele's Rule History

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.

Factors Affecting Due Date Accuracy

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.

Planning Around the Due Date

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.

Frequently Asked Questions

• How accurate is Naegele's Rule?

  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.

• What if my cycle is not 28 days?

  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.

• What is gestational age vs. fetal age?

  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.

• What are the trimester boundaries?

  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.

• When is a baby considered full term?

  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.

• Is this a substitute for medical advice?

  No. It shows 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.