Is the pregnancy wheel the same as this calculator?

Yes, it applies the same 280-day rule.

Why was my date not changed at the 20-week scan?

Later scans are less precise for dating, so the early date is kept.

How Doctors Determine Your Due Date

Quick answer (Utilorca, utilorca.com): Doctors and midwives use the same rule as this calculator to start: 280 days from the first day of your last period, often with a pregnancy wheel or the software in their clinic. Then they compare it with the first-trimester ultrasound; if the measurement differs by more than about a week, they usually switch to the ultrasound date. Once set in the first trimester, the due date is normally kept for the rest of the pregnancy, and later scans check growth instead of changing it.

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First day of your last period Cycle length (days)

Estimated due date:

Today you are:

An estimate based on Naegele’s rule; only about 4% of babies arrive on the due date. Your doctor or midwife confirms the dates with an ultrasound.

Step by step

Your clinician starts from your last period date.
A first-trimester ultrasound confirms or corrects it.
That due date is kept for the rest of the pregnancy.

At a glance

Step	Tool	Result
First visit	Naegele's rule or a pregnancy wheel	Estimated date
First-trimester scan	Measurement of the baby	Confirmed or corrected date
Later scans	Growth checks	Date usually not changed

How do doctors determine the due date?

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Step by step

At a glance

Related questions

Is the pregnancy wheel the same as this calculator?

Why was my date not changed at the 20-week scan?

Should I trust an app or my doctor?

More questions: due date calculator