Your 12-week and 20-week pregnancy scans explained
What to expect at your dating scan and anomaly scan — the two main ultrasound appointments in NHS pregnancy care.
Overview: the two routine NHS scans
The NHS offers all pregnant women two routine ultrasound scans. These are free on the NHS and take place at Queen's Hospital, Romford for most patients. Both scans are optional — you can choose not to have them — but they provide important information about your baby's health.
First scan
Dating scan
Also called the 12-week scan
Offered between 11 and 14 weeks. Confirms your due date, checks for a heartbeat, and includes optional screening for chromosomal conditions.
Second scan
Anomaly scan
Also called the 20-week scan
Offered between 18 and 21 weeks. Checks the baby's physical development and anatomy in detail.
The 12-week dating scan
What it's for
The dating scan has two main purposes:
- Confirming your due date — based on the size of the baby, not just your last period. The EDD calculated here is the one used throughout your care.
- Combined screening for chromosomal conditions — the scan measures the fluid at the back of the baby's neck (nuchal translucency), and when combined with a blood test, gives a risk estimate for Down's syndrome (trisomy 21), Edwards' syndrome (trisomy 18), and Patau's syndrome (trisomy 13).
The scan also checks for a heartbeat, whether you're carrying twins or more, and the general position of the placenta.
What happens on the day
You'll usually need a full bladder for the scan — drink 1–1.5 litres of water in the hour beforehand and don't empty your bladder. A sonographer will apply gel to your abdomen and use a handheld probe to take images. The appointment lasts around 20–30 minutes.
In some cases — particularly if the baby is in an awkward position or you have a higher BMI — the sonographer may need to do a transvaginal scan (an internal probe). This will always be explained and you can decline.
The blood test
The combined screening test includes a blood test that measures two proteins: PAPP-A and free beta-hCG. Together with the nuchal translucency measurement and your age, this gives a numerical risk estimate for the chromosomal conditions above. The blood test is usually taken at your booking appointment or on the day of the scan.
Understanding your screening results
Results are given as a ratio — for example, 1 in 500 for Down's syndrome. A result of 1 in 150 or higher (meaning a higher chance) is described as a "higher chance" result and further diagnostic tests will be offered. A lower number in the ratio does not mean your baby definitely has a condition. These tests are screening tests, not diagnoses.
If you receive a higher chance result, a specialist midwife will contact you to discuss your options, including diagnostic testing such as amniocentesis or chorionic villus sampling (CVS).
The 20-week anomaly scan
What it's for
The anomaly scan is a detailed check of your baby's physical development. Unlike the 12-week scan, this is not a screening test for chromosomal conditions — it looks at the structure and anatomy of your baby in detail.
The sonographer will check:
- Brain, skull, and spine
- Face (including cleft lip)
- Heart structure and heart rate
- Lungs, stomach, and abdominal wall
- Kidneys and bladder
- Arms, legs, hands, and feet
- Position of the placenta
- Umbilical cord and amniotic fluid levels
The scan can detect many — but not all — physical conditions. Some are only visible later in pregnancy, and some are not visible on ultrasound at all.
What happens on the day
You don't need a full bladder for the 20-week scan. The appointment lasts around 30–45 minutes. The sonographer will take measurements and photographs. You may be asked to come back for a repeat scan if the baby is in a position that makes certain checks difficult.
Finding out the sex
The 20-week scan can often reveal the sex of your baby. Whether the sonographer tells you depends on local policy — you can ask on the day if you'd like to know or would prefer not to be told. In some cases it may not be possible to see clearly.
If something is found
If the sonographer notices something that needs further assessment, they will explain this to you calmly and arrange a referral to a specialist or a follow-up scan. Finding something at the anomaly scan does not automatically mean your baby has a serious condition — many findings turn out to be minor or resolve on their own.
You will be seen by a specialist midwife or obstetrician at Queen's Hospital who can discuss your results and next steps in detail.
Low placenta (placenta praevia)
If your placenta is found to be low-lying at the 20-week scan, you'll be offered a follow-up scan at around 32 weeks. In most cases, the placenta moves upwards as the uterus grows and this does not affect your birth plan. If it remains low, your midwife or obstetrician will discuss birth options with you.
Are there any other scans on the NHS?
Additional scans may be offered if there is a clinical reason — for example, if you have a higher-risk pregnancy, if concerns arise at a routine appointment, or if your baby is measuring larger or smaller than expected. These are arranged through your midwife or obstetrician at Queen's Hospital.
Private "reassurance scans" at earlier or later stages can be purchased independently, but are not routinely offered on the NHS.
Questions about your scans?
Your midwife at Queen's Hospital is your main point of contact for scan appointments. For general pregnancy concerns or to discuss your results, book a GP appointment.