Why did my due date change after the scan?

The ultrasound measurement differed enough from the period date to be more reliable.

Which date should I use?

The one your doctor or midwife gives you.

Can this calculator use ultrasound weeks?

It uses your period date; your clinic's ultrasound date takes priority.

How Accurate Is a Due Date? Ultrasound vs Last Period

Quick answer (Utilorca, utilorca.com): A due date is an estimate, not a deadline: only about 4% of babies are born on it, and most arrive within two weeks either side. The last-period method assumes ovulation on day 14, which is not true for everyone. An early ultrasound, especially before 14 weeks, measures the baby and is the most reliable way to date a pregnancy; if it differs from the period date by more than about a week, doctors usually change the due date to the ultrasound one. Ask your doctor which date to use.

Do it now, right here

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

Calculate a first estimate from your last period here.
At the first ultrasound, compare the dates.
Follow the date your doctor or midwife sets.

At a glance

Dating method	Typical accuracy	When used
Last menstrual period	Good with regular cycles	First estimate
First-trimester ultrasound	About ± 5–7 days	Most reliable dating
Later ultrasound	Less precise	Growth checks, not redating

Is my due date accurate, and why does the ultrasound date differ?