Naegele's Rule: The Mathematical Formula Behind Your Pregnancy Due Date

When you visit an obstetrician for the first time, one of the primary pieces of information they provide is your Estimated Due Date (EDD).

While ultrasound measurements will eventually provide a more precise estimation, the initial due date you are given is actually calculated using a standard mathematical formula created over 200 years ago.

This calculation is known as Naegele's Rule.

What is Naegele's Rule?

Named after German obstetrician Franz Karl Naegele, the rule standardizes human gestation to exactly 280 days (40 weeks).

Because most women do not know the exact day of conception, but usually do know when their last menstrual cycle began, the formula uses the First Day of the Last Menstrual Period (LMP) as its anchor point.

The Standard Formula

Here is exactly what your doctor is calculating:

1. Start with the first day of your LMP.
2. Add 7 Days.
3. Subtract 3 Months.
4. Add 1 Year (if necessary to roll into the next calendar year).

A Worked Example:

• LMP: September 9, 2026
• Add 7 Days: September 16, 2026
• Subtract 3 Months: June 16, 2026
• Add 1 Year: June 16, 2027 (This is your Estimated Due Date)

Mathematically, subtracting 3 months and adding 7 days is precisely equal to adding 280 days to the calendar date!

Adjusting for Irregular Cycles

Naegele's rule is brilliant in its simplicity, but it has one massive flaw: It assumes a perfect 28-day menstrual cycle.

If your cycle is longer or shorter than 28 days, ovulation (and therefore conception) happened at a different time relative to your LMP, and the standard formula needs to be modified.

The Adjusted Formula:

• EDD = LMP + 280 days + (Cycle Length - 28 days)

If your cycle is usually 35 days long, you ovulated a week later than the "standard" woman. Therefore, your calculated due date needs to be pushed back by exactly 7 days to remain biologically accurate.

If your cycle is 24 days, you ovulated 4 days early, so the due date must be moved up by 4 days.

IVF Transfer Calculations

If you conceived via In Vitro Fertilization (IVF), Naegele's Rule relies on completely different anchors. Since the exact date of conception is known via the laboratory transfer, the math is hyper-accurate.

Calculations are based on the age of the embryo on the transfer date:

• Day 3 Embryo Transfer: EDD = Transfer Date + 263 days
• Day 5 Embryo Transfer: EDD = Transfer Date + 261 days

The Reality of the "Due Date"

It is vital to remember that an EDD is exactly that: an estimate.

In reality, fewer than 5% of babies are born precisely on their calculated due date. Medically, a pregnancy is considered "full term" anywhere between 37 weeks and 42 weeks of gestation.

You can use our clinical-grade Pregnancy Due Date calculator below to crunch the numbers exactly as your OB/GYN does—complete with adjustments for irregular cycle lengths!