What Is a Dating Scan, and How Accurate Is It?
How a scan works out how many weeks pregnant you are, why it beats counting from your last period, and how a private dating scan fits alongside your NHS 12 week scan.
Quick Answer
A dating scan measures your baby to work out how many weeks pregnant you are and when your baby is due. In the first trimester babies grow at a very similar rate, so a scan is usually accurate to within about a week, and more reliable than counting from your last period. The NHS dating scan is usually between 10 and 14 weeks. A private dating scan gives you a due date sooner, from 9 weeks, and sits alongside your NHS care rather than replacing it. See all our early scans from 6 to 14 weeks.
How a Dating Scan Works
Your sonographer measures your baby and turns that size into weeks and days. The key measurement is the crown-rump length: from the top of the head to the bottom. In early pregnancy babies grow at a very similar, predictable rate, so that one measurement gives a dependable gestation and an estimated due date.
Counting from your last period assumes a 28 day cycle with ovulation around day 14. If your cycles are longer or irregular, you are unsure of the date, or you conceived soon after stopping contraception, your period date can be out by a week or more. A dating scan removes the guesswork.
How Accurate Is a Dating Scan?
A first trimester dating scan is usually accurate to within about a week, and often closer. That is why it is the standard way to date a pregnancy in the UK, and why the scan date usually replaces the period date when the two disagree.
Accuracy depends on timing. Very early, at 6 or 7 weeks, the baby is only a few millimetres long, so a small difference in measurement means a bigger difference in days. From around 9 weeks the baby is big enough to measure clearly, which is why our private dating scan runs from 9 to 11 weeks. Later in pregnancy babies grow at different rates, so scans become less useful for dating.
If your NHS scan gives a due date a few days different from your private scan, that is normal. Your NHS team will use its own measurement from then on.
NHS Dating Scan vs a Private Dating Scan
The NHS dating scan is an important clinical appointment, and you should always attend it. A private scan gives you a due date earlier and lets up to 9 guests share the moment, but it does not include the NHS screening tests.
| NHS dating scan | Private dating scan | |
|---|---|---|
| When | Usually 10 to 14 weeks | 9 to 11 weeks |
| How you book | Arranged through your midwife | Book direct, no GP referral |
| Due date | Yes | Yes |
| Heartbeat and number of babies | Yes | Yes |
| Screening for Down's, Edwards' and Patau's syndromes | Offered (the combined test), if you choose it | Not included |
Under 9 weeks? An Early Viability Scan at 6 to 8 weeks confirms the pregnancy and heartbeat. Already 12 weeks? The Early Reassurance Scan covers 12 to 14 weeks. If you are thinking about screening, the NIPT Test is available from 10 weeks.
Bleeding or Pain? Contact the NHS First
A private scan is for reassurance, not urgent care. If you have bleeding, tummy or shoulder tip pain, dizziness or feel faint, contact your GP, NHS 111 or your local Early Pregnancy Assessment Unit straight away.
Book a Dating Scan in Gosforth
Our Early Dating Scan (£78) runs from 9 to 11 weeks, with a due date, a heartbeat check and images to take home. No GP referral needed. Not sure of your dates? See which early scan to book.
Frequently Asked Questions
A dating scan is an ultrasound that works out how many weeks pregnant you are. The sonographer measures your baby from head to bottom (the crown-rump length) and uses that size to calculate your gestation and an estimated due date, rather than relying on the date of your last period.
The NHS dating scan is usually between 10 and 14 weeks, and is arranged through your midwife. At the same appointment you are offered screening for Down's, Edwards' and Patau's syndromes, if you choose to have it. Always attend it, even if you have had a private scan.
In the first trimester babies grow at a very similar rate, so measuring your baby is the most reliable way to date a pregnancy, usually to within about a week and often closer. It is more reliable than counting from your last period, and if the two disagree the scan date is usually the one used.
Yes, it can. Your NHS team carries out its own dating scan and may confirm or adjust the due date from a private scan. A small change of a few days is normal and is not a sign that anything is wrong.
No. Our Early Dating Scan gives you a due date sooner, from 9 weeks, but it does not include nuchal translucency or combined screening. Keep your NHS appointment.
Usually not. By 9 to 11 weeks your baby can normally be measured over the tummy. In some cases a transvaginal scan is recommended for the clearest view, at no extra cost, and your sonographer will talk it through with you first. Every sonographer at our Gosforth clinic is female.