Dating scan
Confirms the pregnancy, checks the heartbeat, and sets the due date — which every later scan is measured against.
When: Usually 6 to 11 weeks
Examination — looks at how things are
Also called: Early pregnancy scan · Viability scan
What it is
The first scan does three simple things: confirms the pregnancy is in the womb, checks for a heartbeat, and measures the baby to work out how many weeks along you are.
Dating is the part that matters most later. Every scan after this one — the NT scan window, the anomaly scan, growth measurements — is judged against how many weeks pregnant you are. If the dates are wrong, everything measured against them is read against the wrong yardstick. Early measurement is the most accurate there is, which is why this is done first.1
It also finds out how many babies there are, which changes the whole plan for the pregnancy.
Why it is done early
The baby grows at a very predictable rate in the first weeks, so measuring early gives the most reliable due date. Later in pregnancy babies vary much more in size, and a scan then can only estimate dates roughly.
If no heartbeat is seen
Very early on this can simply mean it is too soon to see one, and a repeat scan a week or two later is often what is advised rather than any conclusion being drawn. The doctor will explain what is and is not known at that point.
No scan or screening test can rule out every condition. What each one looks at, what it found, and what it means for you is a conversation with the doctor who performed it — this page explains the tests, it does not interpret your results.
References
- Salomon LJ, Alfirevic Z, Bilardo CM, Chalouhi GE, et al.. ISUOG practice guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound in Obstetrics & Gynecology. 2013;41(1):102-13. PMID 23280739. doi:10.1002/uog.12342The international guideline on the first-trimester scan, including the 11 to 13 weeks 6 days window in which nuchal translucency can be measured and dating is most accurate.
Sources are listed for the specific statements they support. They are published research and guidance, not a substitute for a consultation — what applies to you is decided by a doctor who has assessed you.