Translated from DE

Due date calculation — how accurate is it and when will my baby really arrive?

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Due Date — an Estimate, Not a Promise!

In a nutshell: Only 4% of all babies arrive on their calculated due date. Most arrive within a window of weeks 38-42. Your "due date" is a MIDPOINT, not a deadline.

Calculation (Naegele's Rule)

Formula: First day of last period + 7 days - 3 months + 1 year

Example: Last period January 1, 2026 → January 1 + 7 days = January 8 → January 8 - 3 months = October 8, 2026

How Accurate Is the Due Date?

Time Frame	Probability
On the calculated day	4%
± 1 week	26%
± 2 weeks	50%
Weeks 37-42 (term range)	80%
Before week 37 (early birth)	8-10%
After week 42 (overdue)	5%

Why Is the Due Date So Inaccurate?

Cycle variation: Naegele's Rule assumes a 28-day cycle (many women have longer cycles)
Ovulation timing: Not always day 14 — can be day 10-20
Implantation: Takes 6-10 days after fertilization
Individual maturation time: Some babies simply need more time

More Accurate Methods

Ultrasound weeks 8-12: Most accurate method (± 5 days)
IVF/ICSI: Exact fertilization date known (± 1-2 days)
Cycle tracking: If you tracked your ovulation

What You SHOULD Know

Weeks 37+0 to 41+6 = "on time" (normal)
From week 40+0: Close monitoring (CTG, ultrasound)
From week 41+0 to 41+3: Induction discussed (depending on clinic)
From week 42+0: Induction recommended (increased risk for baby)
First-time mothers are on average 8 days PAST their due date

The Most Important Tip

Tell people outside your inner circle a later date. If your due date is October 15, say "late October." This way you'll avoid the daily "Has it happened yet?!"-calls starting in week 38.

Use our for a quick calculation!

This information does not replace medical advice.

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Not a substitute for medical advice