How Pregnancy Due Date Is Calculated

Your "due date" is actually an estimate. Here's how it's calculated and what it really means.

The Standard Method: Naegele's Rule

• Take the first day of your last menstrual period (LMP)
• Add 1 year
• Subtract 3 months
• Add 7 days

Example: LMP January 1 → Due date October 8

The Assumptions

This method assumes:

• 28-day menstrual cycle
• Ovulation on day 14
• Pregnancy lasting exactly 280 days (40 weeks)

Reality: Most women don't have exactly 28-day cycles.

Ultrasound Dating

First-trimester ultrasound is more accurate because:

• Measures fetal size directly
• Not dependent on cycle length
• Most accurate at 8-12 weeks

If ultrasound differs from LMP by more than 7 days, the due date is usually adjusted.

What Due Date Really Means

• NOT: The day your baby will definitely be born
• IS: The middle of a window when birth is most likely

Normal Range

• "Term" = 37-42 weeks
• Only 4-5% of babies arrive on their due date
• 80% arrive within 2 weeks of due date

When Due Dates Change

• Early ultrasound measurements
• IVF (exact conception date known)
• Irregular cycles discovered

Think of your due date as a target month, not a target day.