The 20-week scan is the appointment most people look forward to and the one they understand least. It is booked as the moment you see the baby properly, and it is actually a screening examination working through a fixed checklist of eleven named conditions while you watch.
Those two things sit uneasily together, which is why the room can feel strange: you are having an experience and the sonographer is doing a job, and the job requires them to stop talking. This guide covers what is on the checklist, what else gets measured, what the scan cannot see, and what happens with each kind of finding.
The short answer: the anomaly scan is done between 18 and 21 weeks and screens for eleven specific conditions, from anencephaly and open spina bifida to serious cardiac abnormalities and Edwards' and Patau's syndromes. It also checks the baby's growth, the position of the placenta and blood flow in the uterus. It is a defined checklist rather than a general health check, and the NHS states directly that it cannot find all conditions — some are much easier to see than others.
The eleven conditions
These are the ones the screening programme is designed to detect. They were chosen because each is serious, each can be seen on ultrasound at this stage, and finding each one changes what happens next.
| Condition | What it affects |
|---|---|
| Anencephaly | The skull and brain fail to form fully |
| Open spina bifida | The spinal column does not close completely |
| Cleft lip | A gap in the upper lip, sometimes involving the palate |
| Diaphragmatic hernia | A gap in the diaphragm lets abdominal organs into the chest |
| Gastroschisis | The bowel develops outside the abdomen, without a covering |
| Exomphalos | Abdominal organs develop outside the body within a sac |
| Serious cardiac abnormalities | Structural heart defects — the hardest group to detect |
| Bilateral renal agenesis | Neither kidney forms |
| Severe skeletal dysplasia | Bones and cartilage do not develop normally |
| Edwards' syndrome | Trisomy 18, usually with several visible findings |
| Patau's syndrome | Trisomy 13, likewise |
Edwards' and Patau's appear on this list because they are also screened for at 11 to 14 weeks. If the combined test was not done in time, the anomaly scan is where they are looked for instead.
What else gets measured
Alongside the checklist, three things are assessed that have nothing to do with the eleven conditions and often matter more to the rest of your pregnancy.
📐 The other half of the appointment
- Growth — head circumference, abdominal circumference and femur length, which become the baseline every later growth scan is compared against.
- The placenta's position — specifically whether it is covering or close to the cervix, which decides whether you need another scan later.
- Blood flow in the uterus — an early signal of how well the placenta is likely to perform in the third trimester.
The measurements are the reason this appointment matters long after it is over. Our guide to reading an ultrasound report explains what HC, AC and FL mean on the printout, and fetal growth percentiles covers how those numbers are turned into a position on a chart.
Soft markers, and why they cause so much trouble
A soft marker is a minor finding that is statistically more common in babies with a chromosomal condition but is also present in many healthy babies. An echogenic focus in the heart, a choroid plexus cyst, a slightly dilated renal pelvis, an echogenic bowel.
None of these is a defect. They are findings that shift a probability slightly, and how much they shift it depends entirely on what else is known. A single soft marker in a pregnancy with a low-chance first trimester screening result usually changes nothing at all. Several together, or one alongside a higher-chance result, is what leads to a referral.
Ask what the marker means given your screening result. A soft marker reported without that context is close to meaningless and is one of the most common causes of unnecessary distress in the second trimester. The right question is not "is this bad" but "does this change my chance figure, and by how much".
What the scan cannot do
Being clear about this is not pessimism; it is what makes a normal result interpretable.
- It cannot find every condition. The checklist is eleven items long, and the NHS says explicitly that some conditions are seen more clearly than others.
- Heart defects are the hardest. The fetal heart is the size of a grape and moving, and some defects only become apparent after birth when the circulation changes.
- It says nothing about most genetic conditions. Those are the domain of NIPT and diagnostic testing, not of imaging.
- It cannot predict how a child will develop. A normal scan describes structure at 20 weeks and nothing else.
- Image quality varies. A higher BMI, an anterior placenta, scar tissue or an awkward fetal position all reduce what can be seen, and sometimes a repeat appointment is arranged simply because the views were incomplete.
Getting the most out of the appointment
Decide about the sex before you go in. Not whether you want to know, but whether you want to be told today — the answer determines when in the appointment to raise it, and it should be at the start.
Ask where the placenta is. It is the finding most likely to generate another appointment, and it is easy to leave without having asked.
Ask whether all the views were obtained. An incomplete scan is common and unremarkable, but it usually means you will be recalled, and it is better to expect that letter than to be alarmed by it.
Bring someone, or do not. This is the appointment where most units allow a partner and where the sonographer is least able to chat. Both of those are worth knowing in advance.
When to call your midwife
Call the same day if: you have vaginal bleeding, a gush or trickle of fluid, severe or one-sided abdominal pain, a severe headache with visual disturbance, sudden swelling of the face and hands, or a fever. After 24 weeks, also call the same day if your baby's movements slow or change — a normal anomaly scan does not change that instruction in any way.
A reassuring 20-week scan describes the baby's structure on the day it was done. It is not a certificate covering the next twenty weeks, and the reasons to ring between appointments are exactly the same afterwards as before. Our full schedule of tests and scans covers what comes next, and the guide to kick counts and baby movements covers the one signal only you can monitor.
Frequently Asked Questions
When is the 20-week scan done?
Usually between 18 and 21 weeks, and in some circumstances up to 23 weeks. The window exists because the organs being examined are large enough to assess reliably by 18 weeks but the baby is still small enough to see in one field of view. A scan much earlier than 18 weeks cannot complete the checklist.
What conditions does the 20-week scan look for?
Eleven specific conditions: anencephaly, open spina bifida, cleft lip, diaphragmatic hernia, gastroschisis, exomphalos, serious cardiac abnormalities, bilateral renal agenesis, severe skeletal dysplasia, Edwards' syndrome and Patau's syndrome. Growth, the position of the placenta and blood flow in the uterus are also assessed. It is a defined checklist, not a general survey of health.
Can the 20-week scan miss something?
Yes, and the NHS says so plainly: the scan cannot find all conditions, and some are seen far more clearly than others. Heart defects in particular are harder to detect than a spinal or abdominal wall defect. A normal scan substantially lowers the chance of a serious structural problem; it does not exclude one.
Why did the sonographer go quiet during my scan?
Because the checklist requires concentration and a specific sequence of views, several of which are difficult to obtain. Silence during a scan is the sound of someone working, not of someone finding something. If it makes you anxious, say so at the start — most sonographers will tell you upfront that they will be quiet while measuring and will talk you through it afterwards.
What is a soft marker?
A minor finding that is more common in babies with a chromosomal condition but is also seen in many entirely healthy babies — an echogenic bowel, a choroid plexus cyst, a slightly dilated kidney. On its own, in a pregnancy with low-chance screening, most soft markers change nothing. What matters is the combination: several together, or one alongside a higher-chance screening result, is what leads to a referral.
What happens if my placenta is low-lying?
You will be offered another scan later in pregnancy, usually around 32 to 36 weeks. Most low-lying placentas at 20 weeks move upwards as the uterus grows and are no longer low by the third trimester. It is the single most common reason the 20-week scan generates an extra appointment, and in most cases the extra appointment resolves it.
Will they tell me the sex at the 20-week scan?
It depends on your hospital's policy — finding out the sex is not part of the screening programme and some units will not report it. If you want to know, say so at the beginning of the appointment rather than at the end, because the clinical checks come first and the baby's position can change while they are done. Ask the sonographer how confident they are — a clear view and a fairly sure guess are different statements.
What if something is found?
You will be told at the appointment rather than left to wait for a letter, and referred on — usually to a fetal medicine unit, often within days. Many findings are minor or need only monitoring. For the serious ones, being found now is what allows a delivery plan, the right hospital and the right team to be arranged in advance, which is the entire purpose of scanning at 20 weeks rather than at 30.
The 20-week anomaly scan — at a glance
The anomaly scan happens between 18 and 21 weeks and works through a fixed checklist of eleven conditions: anencephaly, open spina bifida, cleft lip, diaphragmatic hernia, gastroschisis, exomphalos, serious cardiac abnormalities, bilateral renal agenesis, severe skeletal dysplasia, Edwards' syndrome and Patau's syndrome. It also measures growth, checks where the placenta is lying and assesses blood flow in the uterus. It is a screening examination rather than a general health check, and it cannot find every condition — cardiac defects in particular are hard to see. Soft markers are minor findings that shift a probability rather than diagnose anything, and they should always be interpreted alongside your first trimester screening result. A low-lying placenta is the most common reason for a further scan, and most have moved up by the third trimester.
Inside Baby Novum: the ultrasound log stores estimated fetal weight, head circumference, abdominal circumference and the doctor's comment alongside a photo of the report, and plots each against WHO / INTERGROWTH-21st percentile bands with the FIGO 2021 mean overlaid. Growth charts appear once there are two records, so a follow-up scan reads as a curve rather than as a second isolated number.