How Accurate is Naegele's Rule for Calculating Your Due Date?
When you first discover you are pregnant, the very first question everyone asks is: "When are you due?"
To answer this, your doctor or midwife will likely use a simple mathematical formula called Naegele's Rule. But how exactly does this 200-year-old formula work, and more importantly, how accurate is it?
What is Naegele's Rule?
Named after Franz Karl Naegele, a German obstetrician who published the rule in roughly 1812, this formula calculates your Estimated Due Date (EDD) based on your menstrual cycle.
The math is straightforward:
- Determine the first day of your Last Menstrual Period (LMP).
- Add 7 days.
- Subtract 3 months.
- Add 1 year.
Essentially, the rule assumes that a standard human pregnancy lasts 280 days (40 weeks) from the LMP. It also assumes that every woman has a perfect 28-day menstrual cycle and ovulates exactly on day 14.
The Flaw in the Formula
The biggest issue with Naegele's Rule is that it relies on a perfectly "average" 28-day cycle.
If your cycle is usually 35 days long, you likely don't ovulate on Day 14. You probably ovulate around Day 21. If a doctor uses standard Naegele's Rule on you, they will assume you got pregnant a full week earlier than you actually did. This can lead to a miscalculated due date, which causes unnecessary stress when you inevitably go "overdue" at the end of your pregnancy.
Adjusting the Math
If you know your cycle length, you can adjust Naegele's Rule. For every day your cycle is longer than 28 days, you push your due date back by one day.
To do this math instantly, you can use our Pregnancy & Due Date Calculator. Simply input your LMP and your actual average cycle length, and the calculator will give you a highly personalized EDD.
Track your health accurately: Try our free Pregnancy & Due Date Calculator to get instant, personalized medical math for your pregnancy journey.