The 20-week anomaly scan: what it checks and what to expect
Around the halfway point of pregnancy, you will be offered what is commonly called the 20-week scan, or the anomaly scan. It is one of the most detailed checks in the whole antenatal programme, and for many parents it is also an exciting opportunity to see how much the baby has grown. This article explains what the scan is designed to look for, how to prepare, and what may happen if the sonographer spots something that needs further assessment. All information draws on NHS guidance.
When the scan is offered and what it is for
The anomaly scan is typically offered between 18 and 21 weeks of pregnancy. Its primary purpose is to examine the baby's physical structures in considerable detail, checking whether the anatomy is developing as expected. It is sometimes called a structural scan for this reason. The appointment usually takes around 30 to 45 minutes, though the exact duration depends on the baby's position and how cooperative they are feeling on the day. A cold gel is applied to your abdomen to help the ultrasound probe make good contact with the skin.
It is worth understanding from the outset that the anomaly scan is a screening tool rather than a diagnostic one. A normal result is genuinely reassuring and significantly lowers the likelihood of an undetected structural issue, but no scan can guarantee that every aspect of the baby's development is typical. The NHS is clear that some conditions are beyond what ultrasound can reliably detect.
What the sonographer checks
The sonographer works through a detailed checklist of structures during the appointment. Each area of the baby's body is assessed in turn, and the sonographer will move through the examination methodically. The structures reviewed include:
- Brain and skull: the shape and development of the brain is assessed, along with the bones of the skull
- Face: where image quality allows, the sonographer looks for signs of a cleft lip
- Spine: the vertebrae and the skin covering the spine are checked along its full length
- Heart: the four chambers of the heart are examined, along with the major blood vessels leaving it
- Stomach: a visible stomach on the scan suggests the baby has been swallowing amniotic fluid, which is a positive sign
- Kidneys: both kidneys are checked for their presence and appearance
- Abdominal wall: the sonographer checks that the organs are contained within the abdominal wall rather than protruding outside it
- Limbs and fingers: the arms, legs and digits are reviewed
In addition to the baby's anatomy, the scan also looks at where the placenta is positioned in the uterus. If the placenta is found to be low-lying at this stage, you will be offered a further scan at around 32 weeks to check whether it has moved as the uterus has expanded.
Preparing for your appointment
Having a reasonably full bladder can be helpful earlier in the second trimester, as fluid helps improve the quality of ultrasound images. However, a full bladder is considerably less critical at 18 to 21 weeks than it is at the 12-week scan, when the uterus is still lower in the abdomen. Your scan invitation may include specific instructions from your unit, so it is worth reading those in advance. Wearing clothing that gives easy access to your abdomen will also make the appointment more comfortable.
You are usually welcome to bring a birth partner or support person with you. If you would like to bring a young child, it is worth checking with your unit beforehand, as policies can vary. The sonographer's focus throughout the appointment is the clinical examination, so there may be periods of quiet while they concentrate on measurements and images.
Finding out the baby's sex
The 20-week scan is often the point at which parents choose to find out whether they are expecting a boy or a girl. Whether the sex can be seen depends on the baby's position during the scan. If you do want to know, you can tell the sonographer at the start of the appointment. If you would prefer not to know, or if you have mixed feelings in the room, let them know that too. It is also worth being aware that not every NHS unit routinely offers sex identification, so checking with your hospital or unit in advance is a sensible step.
If something unexpected is found
For most people, the anomaly scan passes without any concerns being raised, and this is the most common outcome. However, if the sonographer identifies something that requires a closer look, they will tell you during the appointment or shortly afterwards. They will explain as clearly as they can what they have seen and why it needs further assessment.
Depending on what is found, the next step may be a more detailed ultrasound with a specialist, a referral to a fetal medicine consultant, or a discussion about whether genetic testing might be appropriate. The team supporting you will make sure you have access to information and, importantly, to emotional support. A finding at the anomaly scan does not automatically mean a serious diagnosis: some differences spotted at this stage resolve on their own, and others may be minor variations that need monitoring rather than treatment. Whatever the situation, you will not be left to work through it without guidance.
Frequently asked questions
What does the 20-week scan check?
The anomaly scan looks in detail at the baby's anatomy, including the brain, spine, heart, stomach, kidneys, limbs and face. It also checks the position of the placenta. The aim is to identify structural differences that may need monitoring, treatment or further investigation.
Can the 20-week scan miss things?
The scan significantly reduces the chance of an undetected structural condition, but it cannot detect everything. Image quality depends on the baby's position, the amount of amniotic fluid, and maternal body factors. A normal result is reassuring but is not a guarantee that all structures are typical.
Will I find out the sex at the 20-week scan?
The sex can often be seen at 20 weeks, depending on the baby's position. You can let the sonographer know in advance whether you would like to be told. Not all NHS units routinely offer sex identification, so it is worth checking beforehand.
What happens if the scan finds something unexpected?
If the sonographer sees something that needs further assessment, they will explain what they have found and arrange a referral to a specialist or a more detailed scan. They will also ensure you have information and support to help you understand what any finding means.
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