
07/22/2025
📚 Where the 40-Week Due Date Comes From
The 40-week (280-day) pregnancy calendar is based on a formula known as Naegele’s Rule, developed by Franz Karl Naegele, a German obstetrician in the early 1800s. He suggested that a woman’s estimated due date (EDD) could be calculated as:
First day of last menstrual period (LMP) + 1 year – 3 months + 7 days
This method assumes a 28-day menstrual cycle with ovulation occurring on day 14. Under this assumption, pregnancy lasts 280 days (40 weeks) from the first day of the last period.
⸻
🧠 Why Do We Still Use It?
Even though we now know it’s not perfectly precise, we still use the 40-week model for several practical and historical reasons:
1. Simplicity & Standardization: It gives doctors, midwives, and researchers a standard timeline to track fetal development and schedule prenatal care.
2. Clinical Guidelines & Insurance: Everything from prenatal testing to maternity leave is built around the 40-week model in most medical systems.
3. Most Pregnancies Fall Close: While only about 4–5% of babies are born on their due date, most are born between 37 and 42 weeks, so it provides a general framework.
4. Technological Anchoring: Ultrasound dating early in pregnancy often adjusts the EDD, but it’s still compared against the original 40-week calendar.
⸻
❗️But It’s Flawed
• It doesn’t account for ovulation variability—many women don’t ovulate on day 14.
• It ignores individual cycles—some are shorter or longer than 28 days.
• First-time moms often go past 40 weeks.
• Repeat moms might deliver earlier.
• The model is based on European 19th-century data, not today’s diverse populations or real-life birth trends.
⸻
🌕 Fun Fact
Studies and anecdotal data (like midwives often observe) show more babies arrive during full moons than not—and the average actual pregnancy length for first-time moms is closer to 41 weeks and 1 day.
⸻
⚖️ Summary
We still use the 40-week due date calendar mostly for consistency, not precision. It’s a helpful planning tool, but not a biological clock—and due dates should always be seen as an estimate, not an expiration date.