What is the 12-week scan for?

The 12-week scan is the dating scan, and its main job is to work out how far along you are. The NHS says that in England you will be offered it at around 10 to 14 weeks, and that it checks:

how many weeks pregnant you are, and your estimated due date
whether you are expecting more than one baby
that the baby is growing in the right place
your baby's development — the NHS notes it can detect some conditions, such as spina bifida

NICE guideline NG201, which sets the standard for antenatal care in England, puts the window more precisely: a scan "between 11+2 weeks and 14+1 weeks" to determine gestational age, detect a multiple pregnancy and, if you have opted for it, screen for Down's, Edwards' and Patau's syndromes. If you are carrying more than one baby, how early a scan can show twins covers what the sonographer looks for.

What happens at the 12-week scan?

Most dating scans happen in your local hospital's ultrasound department and are carried out by a sonographer. The NHS says the appointment usually takes about 20 minutes, and that you may need a full bladder because it makes the image clearer — ask your midwife beforehand.

The room is dimly lit so the sonographer can get good images. Gel goes on your tummy and the sonographer moves a probe over it, sometimes pressing a little to get a good view. If your baby is lying awkwardly, the NHS says the scan may take longer or have to be repeated at another time.

You can bring someone with you, but most hospitals do not allow children. If you want a printed picture, the NHS says to check whether your hospital provides them; if it does, there may be a charge.

How the scan works out your due date

The sonographer measures your baby from the top of the head to the bottom, the crown–rump length (CRL), and converts that into weeks and days. Your due date was first calculated from the first day of your last period; the NHS says it may now be adjusted to match the scan.

Why the scan takes priority: the American College of Obstetricians and Gynecologists (ACOG) states that measuring the embryo or fetus in the first trimester, up to 13 weeks and 6 days, "is the most accurate method to establish or confirm gestational age", with a CRL accuracy of plus or minus 5 to 7 days. For US practice it says the due date should be changed if a scan before 14 weeks differs from the period-based date by more than 7 days, and that before 9 weeks a difference of more than 5 days is reason enough.

The CRL only works up to a point. The NHS screening handbook says that once the CRL is greater than 84.0 mm, gestational age is calculated from the head circumference instead.

Is the combined test done at the 12-week scan?

Usually, if you have said yes to it and you are in the window. The combined test screens for Down's syndrome, Edwards' syndrome and Patau's syndrome using a blood test and a measurement of the fluid at the back of the baby's neck, the nuchal translucency (NT). The NHS says the NT can be taken during the dating scan, so the two usually happen together.

In England the window is set by the baby's size rather than the calendar. The fetal anomaly screening programme handbook states that the combined test can only be done when the CRL is between 45.0 mm and 84.0 mm. Below 45.0 mm you are called back for another scan. Above 84.0 mm you are offered the quadruple test if you meet its criteria: a blood test between 14 and 20 weeks that the NHS says is "not quite as accurate as the combined test" — see quadruple test results. If the NT cannot be measured, at least one more attempt is offered before you are moved to the quadruple test.

The result combines your age, two blood markers (free beta hCG and PAPP-A) and the scan measurements. The handbook says the chance result is usually available within a few days. A higher-chance result — the handbook's range for offering NIPT is between 1 in 2 and 1 in 150 — leads to an offer of further testing, not a diagnosis. Two findings have their own pages: an NT of 3.5 mm or more, which the handbook says should be referred straight away (high nuchal translucency), and low PAPP-A.

If you decline screening

Saying no to the combined test does not switch off the sonographer's eyes. The handbook requires that everyone who accepts a dating scan is told that unexpected findings may be identified, including people who have declined screening.

Can you find out the sex at the 12-week scan?

Not as part of NHS care. The NHS states that finding out the sex "is not offered as part of the national screening programme", and that you may be able to find out at the 20-week scan, depending on your hospital's policy. If anyone offers you a guess at this scan, treat it as provisional: the NHS says that even at 20 weeks a sonographer cannot be 100% certain. When you can find out the sex sets out each method and how accurate it is, and the nub theory page covers the angle people read on 12-week scan photos.

The first-trimester scan outside the UK

United States. ACOG describes first-trimester screening as a blood test and NT scan done together between 10 and 13 weeks, and cell-free DNA testing as available from 10 weeks, with results in about a week. See US pregnancy screening tests.
Australia. Pregnancy, Birth and Baby, the Australian Government service, says the NT scan is done between 11 weeks and 13 weeks and 6 days as part of combined first trimester screening. It notes NIPT is more accurate but "not currently funded by Medicare". See screening in Australia.
New Zealand. Health New Zealand advises having the blood test at 9 to 10 weeks (it can be taken up to 13 weeks and 6 days) and the scan between 12 weeks and 13 weeks and 6 days. "While the blood test is free, you may have to pay for the ultrasound."

If something worries you before the scan

You do not have to wait for the dating scan if you have bleeding or pain. Bleeding in early pregnancy sets out what to do and how fast, and early pregnancy scans explains the scans offered before 10 weeks.

Frequently asked questions

Can you tell if it's a boy or girl at the 12-week scan?

Not on the NHS. Finding out the sex is not part of the national screening programme, and the NHS names the 20-week scan as the point where you may be told, depending on your hospital's policy. Even then, it says the sonographer cannot be 100% certain, for example if your baby is lying awkwardly.

Do I need a full bladder for my 12-week scan?

Possibly. The NHS says you may need a full bladder for the dating scan because it makes the ultrasound image clearer, and suggests asking your midwife or doctor beforehand whether yours needs one. If nobody has told you and you are unsure, ring the ultrasound department before you set off.

Can I get a 12-week scan photo?

Often, but not everywhere. The NHS says you need to check whether your hospital provides pictures and that, if it does, there may be a charge. Ask when you book or before the appointment, so you know whether to bring payment and how many prints are available.

What if my dating scan is after 14 weeks?

The scan still dates your pregnancy, using the head measurement once the baby is beyond the CRL range. You will not be offered the combined test; the NHS offers a blood test between 14 and 20 weeks instead, which screens for Down's syndrome but is not quite as accurate as the combined test.

The 12-Week Scan