How Your Due Date Is Calculated (Naegele's Rule)

The moment a pregnancy is confirmed, the first question is always “when?” Doctors have used the same formula since 1812 — and while it’s not perfect, it’s remarkably practical for estimating that 280-day countdown.

Naegele’s Rule

Due Date = Last Menstrual Period (LMP) + 280 days

Or the shortcut:
Due Date = LMP − 3 months + 7 days + 1 year

Assumptions:
  - 28-day cycle
  - Ovulation on day 14
  - Pregnancy lasts 266 days from conception (280 from LMP)

The formula dates from 1812 when German obstetrician Franz Naegele published it. Despite being over 200 years old, it remains the clinical standard for initial dating.

Worked example

Last menstrual period started: March 10, 2026

Method 1 (add 280 days):
  March 10 + 280 days = December 15, 2026

Method 2 (shortcut):
  March 10 − 3 months = December 10
  December 10 + 7 days = December 17
  (slight variation due to month-length differences)

Estimated Due Date: December 15, 2026

Adjusting for cycle length

If your cycle isn’t 28 days, adjust:

Adjusted Due Date = Naegele's Date + (Actual Cycle Length − 28)

Example: 32-day cycle
Due date = December 15 + 4 days = December 19

Longer cycles mean later ovulation, which pushes the due date forward.

Why due dates are estimates

• Only 4–5% of babies are born on their due date
• “Full term” is a 5-week window: 37–42 weeks
• First-time mothers tend to go past their due date by 3–5 days
• Ultrasound dating (6–13 weeks) is more accurate than LMP for irregular cycles
• Multiple studies suggest 281 days (not 280) is the median for first pregnancies

Pregnancy timeline

Milestone	Weeks	From LMP
Implantation	3–4 weeks	~1 week after missed period
First heartbeat	6 weeks	Detectable on ultrasound
End of first trimester	12 weeks	Major organs formed
Anatomy scan	18–22 weeks	Gender can be determined
Viability threshold	24 weeks	Survival possible with NICU
Full term begins	37 weeks	Safe for delivery
Due date	40 weeks	Statistical midpoint
Post-dates	41–42 weeks	Induction typically offered

When to use this calculation

• First estimate before an ultrasound confirms dates
• Planning maternity leave and birth preparation
• Understanding where you are in the trimester timeline
• Discussing timing with your healthcare provider

Tips for expectant parents

• Don’t fixate on the exact date — think of a “due window” (38–42 weeks)
• First trimester ultrasound can adjust dates by 5–7 days
• If LMP is uncertain, dating ultrasound is more reliable
• “Full term” was redefined in 2013: 39–40 weeks is now considered ideal
• Track from LMP even if you know conception date — medical records use LMP weeks

Estimate your due date and weekly milestones with OurDailyCalc’s pregnancy due date calculator — enter your LMP and cycle length for an adjusted prediction.

TL;DR

• Naegele’s Rule: LMP + 280 days = due date
• Only 4–5% of babies arrive on the exact due date
• Adjust by adding (cycle length − 28) days for non-standard cycles
• Full term spans 37–42 weeks; 39–40 is considered optimal
• First-trimester ultrasound is more accurate than LMP for irregular cycles