Pregnancy Due Date Calculator (Naegele) Formula

Understand the math behind the pregnancy due date calculator (naegele). Each variable explained with a worked example.

Use the Pregnancy Due Date Calculator (Naegele)

Formulas Used

Current Gestational Week

gestational_weeks = floor(lmp_days_ago / 7)

Extra Days

gestational_days_extra = lmp_days_ago - floor(lmp_days_ago / 7) * 7

Days Until Due Date

days_remaining = max(280 - lmp_days_ago, 0)

Weeks Remaining

weeks_remaining = max(280 - lmp_days_ago, 0) / 7

Variables

VariableDescriptionDefault
lmp_days_agoDays Since Last Period Started(days)84

How It Works

Naegele's Rule

The estimated due date is 280 days (40 weeks) from the first day of the last menstrual period (LMP).

Due Date = LMP + 280 days

Trimester Breakdown

| Trimester | Weeks | |---|---| | First | 1-12 | | Second | 13-27 | | Third | 28-40 |

Only about 5% of babies are born on their exact due date. Most arrive within 2 weeks of the estimate.

Worked Example

A woman whose last period started 84 days ago.

lmp_days_ago = 84
  1. 01Gestational week = floor(84 / 7) = 12 weeks
  2. 02Extra days = 84 - 12 x 7 = 0
  3. 03Days remaining = 280 - 84 = 196 days
  4. 04Weeks remaining = 196 / 7 = 28.0 weeks

Ready to run the numbers?

Open Pregnancy Due Date Calculator (Naegele)