Question 1

What is Naegele's rule?

Accepted Answer

Naegele's rule calculates the estimated due date by adding 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. It is the standard method used by healthcare providers worldwide.

Question 2

What if my cycles are irregular?

Accepted Answer

If your cycles are longer or shorter than 28 days, the standard due date calculation may be off. Many providers adjust the due date based on first-trimester ultrasound measurements, which are more accurate than LMP-based calculations, especially with irregular cycles.

Question 3

Does the due date change after an ultrasound?

Accepted Answer

Yes. If your first-trimester ultrasound (before 14 weeks) shows a gestational age significantly different from your LMP-based estimate — typically by more than 5–7 days — your provider may revise the due date to match the ultrasound measurement.

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Why does cycle length matter?

How accurate is this?

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