The short explanation
An early scan is usually a more accurate way to date a pregnancy than the date of the last period. Counting from the last period assumes ovulation happened on a set day of the cycle, but many women ovulate earlier or later, especially with irregular cycles. In early pregnancy babies grow at a very similar rate, so measuring the baby on a scan gives a reliable estimate of how far along the pregnancy is. That is why doctors commonly adjust the due date when the early scan and the period date differ by more than a small margin. A change of about a week is common and does not by itself suggest a problem.
What depends on your situation
The parts of this that change from woman to woman. Uncertainty is stated, not hidden.
- The dating scan done in the first trimester is the most accurate. Later scans are less useful for dating because babies vary more in size as pregnancy goes on.
- The corrected date matters for timing screening tests, judging growth later, and deciding when a pregnancy has gone past its due date.
- If you are unsure of your period date, have irregular cycles or conceived while breastfeeding, the scan date is even more useful.
What this answer cannot determine
The boundary of online education. These need a clinician who can see you.
- This cannot tell you anything about the health of your pregnancy beyond the dating. Your clinician will interpret your scan report as a whole.
Questions for your clinician
Take these with you. You can add any of them to an appointment card.
- What is my new due date, and will you use it from now on?
- Did the scan show anything else I should know about?
- Which screening tests are timed by this date, and when are they?
- Should I keep my old date anywhere in my records?
When to seek help
Clinician-approved guidance for this topic. When in doubt, being seen is never the wrong choice.
Emergency — call 112 now
- Call 112 or go to the nearest emergency department if you have heavy bleeding, severe belly pain or fainting at any point in pregnancy.
Book an appointment soon
- See your doctor within a few days if anything in the scan report was unclear and you were not given a chance to ask.
Raise it at a routine appointment
- Use the corrected due date for your antenatal card and future appointments.
Sources
The evidence this explanation is built on.
- Prenatal care MedlinePlus (NIH)
- Pregnancy MedlinePlus (NIH)
Who wrote and reviewed this
Every AskHer answer names its author. A clinically reviewed answer also names the clinician who signed it off — if no clinician is named below, it has not been clinically reviewed.
- Author
- AskHer Editorial (AI-assisted draft) AskHer's editorial team — not an individual clinician Works in English, தமிழ், हिन्दी
- Clinical reviewer
- Nobody yet No clinician has signed this answer off, and no person on the editorial team has read it yet. That is what the “Written with AI assistance” label means. It is queued for an editor to read, and then for a clinician who works in this area. The sources it was drafted against are listed above.
- Review status
- Written with AI assistance — not yet reviewed by a person
- Last reviewed
- Not yet reviewed by a person
- Version
- 1
Correction history
Substantive changes are listed, not quietly edited away.
No corrections have been made since this answer was first published.
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