In short
The mid-pregnancy scan checks the baby's structure against a defined list of conditions. Knowing what it screens for, what it cannot see, and what soft markers mean makes the report far less frightening to read.
Emergency if: Heavy bleeding, severe abdominal pain, severe headache with visual changes, or fluid leaking with painful contractions — go to your maternity unit or an emergency department now.
On your report
Beta-hCG (Non-Pregnancy / Tumor Marker Use), AFP (Alpha-Fetoprotein), Hemoglobin, TSH +1 more
On this page
01
When it happens and what it is for
The anomaly scan is usually done between 18 and 21 weeks of pregnancy, and in some cases up until 23 weeks. By that stage the organs are formed well enough to examine but the baby is still small enough to view in one sweep, which is why the window is narrow rather than a matter of convenience.
It is a structural scan, not a wellbeing check. The sonographer works through a checklist — head and brain, face and lips, spine, heart and its outflows, abdominal wall, stomach, kidneys and bladder, and the limbs and bones — and also examines the placenta, cord and the volume of amniotic fluid.
02
The conditions it screens for
Screening programmes typically name a specific list of eleven conditions the scan looks for: 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.
Those conditions differ widely in how reliably ultrasound finds them. Some, such as anencephaly, are seen almost every time; serious heart abnormalities are considerably harder and a proportion are not visible at this stage. The scan is also affected by the baby's position, by how far along you are, and by body habitus, and a repeat scan is sometimes needed simply because the baby would not turn.
03
What the scan cannot tell you
A normal anomaly scan is genuinely reassuring but it is not a guarantee of a healthy baby. The scan cannot find all conditions, and some are seen more clearly than others, so a clear report reduces the chance of a structural problem without removing it. Conditions that develop later in pregnancy are, by definition, not visible at 20 weeks.
It also says almost nothing about intellectual development, hearing, vision, or most genetic conditions, which are not structural. Growth problems are a separate question addressed by later growth scans, since a baby measuring normally at 20 weeks can still slow down afterwards.
04
Soft markers and incidental findings
Soft markers are minor appearances that are common in healthy babies but very slightly more frequent when a chromosomal condition is present — an echogenic focus in the heart, a mildly dilated kidney, a choroid plexus cyst. In isolation, in a pregnancy that has already had a low-risk screening result, most soft markers turn out to mean nothing at all.
They are reported because the combination matters: several together, or one alongside a higher screening risk, changes the conversation. If a soft marker is mentioned to you, the useful question is whether it alters your risk estimate at all, and what the plan is — often nothing more than a repeat look in a few weeks.
05
Placenta, fluid, and what comes after
The scan also reports where the placenta has implanted. A low-lying placenta at 20 weeks is common and usually resolves as the uterus grows upward, so it is followed with a repeat scan later in pregnancy rather than acted on immediately. Amniotic fluid that is markedly high or low is the finding that most often prompts closer follow-up.
If something is found, the next step is usually a detailed scan by a fetal medicine specialist, sometimes with a fetal echocardiogram or a discussion about genetic testing. Nothing about that pathway is decided in the scan room, and taking a day to absorb the information before choosing is entirely reasonable.
When to act
Routine — see a doctor
An isolated soft marker or a low-lying placenta: these are usually followed with a repeat scan rather than acted on straight away.
Same-day — call promptly
Reduced or absent fetal movements after 24 weeks, or any vaginal bleeding — contact your maternity unit the same day, whatever the scan said.
Emergency — act now
Heavy bleeding, severe abdominal pain, severe headache with visual changes, or fluid leaking with painful contractions — go to your maternity unit or an emergency department now.
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