Dating Scans Explained: How Your Due Date Is Worked Out
Why measuring your baby beats counting from your last period, how accurate it is, and where a private dating scan fits with the NHS scan.
Quick Answer
A dating scan measures your baby and turns that size into weeks and an estimated due date. Early in pregnancy babies grow at almost the same rate, so a first-trimester scan is usually accurate to within about a week, and often closer. The NHS dating scan is usually at around 10 to 14 weeks; a private Early Dating Scan gives you a date from 9 weeks. Compare all our early pregnancy scans.
Measuring Beats Counting
Counting from your last period assumes a 28 day cycle and ovulation on day 14. Longer or irregular cycles, an uncertain period date, or conceiving soon after stopping contraception can put that date out by a week or more.
A dating scan measures your baby from the top of the head to the bottom, the crown-rump length. Because babies grow at a very similar rate in early pregnancy, that single measurement gives a reliable gestation and due date.
How Accurate Is It?
Usually to within about a week, and often closer. That is why a first-trimester scan is the standard way to date a pregnancy, and why the scan date normally replaces the period date when the two disagree.
Timing matters. At 6 or 7 weeks the baby is so small that a tiny measuring difference shifts the date more. From around 9 weeks the baby is big enough to measure clearly, which is why our dating scan runs from 9 to 11 weeks. Later in pregnancy babies grow at different rates, so scans become less useful for dating.
Private Dating Scan and the NHS Scan
| NHS dating scan | Our Early Dating Scan | |
|---|---|---|
| When | Usually 10 to 14 weeks | 9 to 11 weeks |
| Booking | Through your midwife | Direct, no GP referral |
| Due date and heartbeat | Yes | Yes |
| Screening for Down's, Edwards' and Patau's syndromes | Offered (the combined test) | Not included |
Keep your NHS appointment whatever you book privately. Your NHS team may confirm or adjust your due date by a few days, which is normal. Under 9 weeks? The Early Viability Scan runs from 6 to 8 weeks. Thinking about screening? NIPT is available from 10 weeks.
Symptoms? Contact the NHS Before Booking
Our scans are for peace of mind, not urgent care. Bleeding, tummy pain, pain in the tip of your shoulder, dizziness or feeling faint all need NHS advice first: call your GP, NHS 111 or your local Early Pregnancy Assessment Unit. For heavy bleeding, severe pain or collapse, call 999.
Book a Dating Scan in Gateshead
The Early Dating Scan (£78) runs from 9 to 11 weeks, with a due date, heartbeat check and images. Not sure of your dates? See which early scan to book.
Frequently Asked Questions
The sonographer measures your baby from head to bottom (the crown-rump length). Because babies grow at a very similar rate early on, that measurement gives your gestation and an estimated due date.
A first-trimester dating scan is usually accurate to within about a week, and often closer. It is more reliable than counting from your last period.
Usually at around 10 to 14 weeks, arranged by your midwife. Screening for Down's, Edwards' and Patau's syndromes is offered at the same appointment if you choose.
It may move by a few days, which is normal. Your NHS team uses its own measurement from then on.
No. Our Early Dating Scan gives a due date and checks the heartbeat but does not include nuchal translucency or combined screening. Keep your NHS appointment.