Pregnancy Due Date Calculator

Calculate your baby's due date based on your last menstrual period or conception date. Free, accurate pregnancy calculator with week-by-week information.

Pregnancy Due Date Calculator

Understanding Pregnancy Due Date Calculation: Theory, Usage, and History

What is a Due Date?

A pregnancy due date, also called the estimated date of delivery (EDD), is the predicted date when a baby will be born. It's calculated as 280 days (40 weeks) from the first day of the last menstrual period (LMP). This calculation assumes a regular 28-day menstrual cycle with ovulation occurring on day 14.

Naegele's Rule:

Due Date = LMP + 280 days (or LMP + 1 year - 3 months + 7 days)

For irregular cycles: Adjust for ovulation timing based on cycle length

How to Use This Calculator

  1. Choose calculation method - LMP, conception date, or ultrasound
  2. Enter the relevant date - First day of last period, conception, or ultrasound date
  3. Adjust cycle length if needed - For irregular cycles (21-35 days)
  4. Click "Calculate Due Date" to get your estimated delivery date
  5. Review pregnancy milestones and current gestational age

Important: Due dates are estimates. Only about 5% of babies are born on their exact due date. Most babies are born within 2 weeks before or after the due date.

History of Due Date Calculation

The method for calculating pregnancy due dates was developed by German obstetrician Franz Karl Naegele (1778-1851). Naegele's Rule, established in the early 1800s, became the standard method for estimating delivery dates and is still widely used today.

Naegele based his calculations on the observation that human pregnancy typically lasts about 280 days from the last menstrual period. He developed his rule by studying biblical references and historical records, noting that pregnancy duration was remarkably consistent across different populations and time periods.

Modern obstetrics has refined these calculations with ultrasound technology, which can provide more accurate dating, especially in the first trimester. However, Naegele's Rule remains the foundation for pregnancy dating worldwide.

Calculation Methods

Last Menstrual Period (LMP)

Most common method. Adds 280 days to the first day of your last period. Assumes 28-day cycle with day 14 ovulation.

Conception Date

If you know when conception occurred, adds 266 days (38 weeks) to that date. More accurate for irregular cycles.

Ultrasound Dating

Most accurate method, especially in first trimester. Uses fetal measurements to determine gestational age and calculate due date.

Pregnancy Trimesters

First Trimester (Weeks 1-12)

Major organ development, morning sickness common, first prenatal visits and screenings.

Second Trimester (Weeks 13-26)

Often called the "golden period," anatomy scan around week 20, baby's movements felt.

Third Trimester (Weeks 27-40)

Rapid growth, preparation for birth, regular monitoring, baby considered full-term at 37 weeks.

Factors Affecting Due Date Accuracy

Cycle Irregularity: Women with irregular cycles may ovulate earlier or later than day 14, affecting calculations.

First vs. Subsequent Pregnancies: First-time mothers often deliver slightly later than the due date.

Maternal Age: Older mothers may have slightly longer pregnancies.

Genetic Factors: Family history of early or late deliveries can influence timing.

Multiple Pregnancies: Twins and multiples typically arrive earlier than singleton pregnancies.

Important Pregnancy Milestones

  • Week 4-6: Missed period, positive pregnancy test
  • Week 8-10: First prenatal appointment, heartbeat detection
  • Week 11-14: First trimester screening, nuchal translucency scan
  • Week 18-22: Anatomy scan, gender determination possible
  • Week 24-28: Glucose screening, viability milestone
  • Week 32-36: Regular monitoring increases, baby shower time
  • Week 37-42: Full-term period, ready for delivery

Related Calculators

Quick Tips

  • • Track your menstrual cycle for more accurate calculations
  • • Ultrasound dating is most accurate in first trimester
  • • Only 5% of babies are born on their exact due date
  • • Consult your healthcare provider for personalized care