Your pregnancy due date is officially called the Estimated Date of Delivery (EDD) — the key word being “estimated.” Only about 5% of babies are born on their exact due date. The calculation is useful for tracking fetal development and planning care, but it is not a precise prediction. Understanding how it works helps you have realistic expectations.

Naegele’s Rule: The Standard Calculation

The most common method: Due Date = First Day of Last Menstrual Period + 280 days (40 weeks). This formula was developed by Franz Naegele in 1812 and remains the obstetric standard. It assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, your provider may adjust the calculation accordingly.

First Trimester Ultrasound: The Most Accurate Dating

An ultrasound between weeks 8–14 that measures crown-rump length (CRL) is the most accurate dating method available. If the ultrasound date differs from the LMP-calculated date by more than 7 days, the ultrasound date typically becomes the official EDD. This is why providers often do not change dating for small discrepancies.

Key Pregnancy Milestones from Due Date

• Weeks 1–13: First trimester — embryonic development, highest miscarriage risk period
• Week 10–13: First trimester screening (NIPT, nuchal translucency ultrasound)
• Week 18–22: Anatomy scan (detailed ultrasound, sex determination if desired)
• Week 24: Viability threshold (survival possible with intensive care)
• Week 28: Third trimester begins
• Week 37: Early term — no longer preterm
• Weeks 39–40: Full term — optimal delivery window
• Week 42: Post-term — induction typically recommended

Why Due Dates Are Often “Wrong”

The 280-day calculation is an average. Normal full-term pregnancy ranges from 37–42 weeks — a 35-day window. First-time mothers tend to deliver slightly later on average than women who have given birth before. There is also genuine biological variation in gestational length that no formula can fully predict.