Cycle variability and dating accuracy
The 28-day cycle is an average, not a rule. Here's how a longer or shorter cycle shifts your due date.
3 min readReviewed May 1, 2026
Quick answer
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If yours is 32 days, your true due date is roughly 4 days later than LMP-based math suggests.
What throws LMP dating off
- Cycle longer or shorter than 28 days.
- Irregular cycles or recent hormonal contraception.
- Uncertain or unknown LMP date.
- Late ovulation in an otherwise normal cycle.