Common Fetal Anomalies (CNS, cardiac, GU)
- The mid-pregnancy anatomy scan is a head-to-toe checklist, and three systems generate most of the "uh oh" moments: the brain, the heart, and the kidneys/bladder.
- For CNS, your bread-and-butter signs are ventriculomegaly (the brain's fluid spaces too big), open neural tube defects (the spine that didn't zip shut), and midline problems.
- For the heart, you don't memorize every lesion — you confirm a normal four-chamber view and outflow tracts, and treat anything that breaks symmetry as suspicious.
- For the GU tract, the recurring theme is fluid that can't get out: dilated kidneys (hydronephrosis), a big bladder, and the downstream effect on amniotic fluid.
- Amniotic fluid volume is the silent vital sign — too little or too much is often the first hint something is wrong.
Somewhere around the middle of pregnancy, a sonographer spends a chunk of time gliding a probe across a belly, measuring a person who is roughly the size of a mango and entirely uninterested in holding still. This is the fetal anatomy survey, and it's basically a very high-stakes game of "spot everything." The full checklist is long. But if you learn the three systems that produce most of the findings — central nervous system, heart, and genitourinary tract — you'll understand the lion's share of what makes a sonographer's eyebrows go up.
Let me walk you through them like a friend pointing at a screen, because that's exactly what this is.
The brain: it's mostly about fluid and midlines
Think of the fetal brain as a house that's still being built. Two things you check: are the rooms (the fluid-filled ventricles) the right size, and is the central hallway (the midline) where it should be?
The most common red flag is ventriculomegaly — the lateral ventricles, which carry cerebrospinal fluid (CSF), measured wider than they should be. The ventricle is supposed to be a modest little channel; when it balloons out, that's the brain's plumbing backing up, the same way a sink backs up when the drain clogs. Ventriculomegaly isn't a diagnosis by itself — it's a signpost. It can come from obstruction, from a malformation, or sometimes from nothing serious at all. When it's marked, it shades toward true hydrocephalus.
Then there are the neural tube defects — the spine or skull that didn't fully zip up early on. The classic is spina bifida, an open defect in the lower spine. The sneaky part: you often spot it not by staring at the spine, but by reading the skull. An open spinal defect tugs the hindbrain downward, and that gives you two famous cranial signs.
The "lemon" and "banana" signs are skull and cerebellum shape clues for an open neural tube defect. The skull frontal bones scallop inward like a lemon, and the cerebellum gets pulled and curved into a banana shape with effacement of the fluid space behind it. A fruit bowl that means "go look hard at the spine."
The third midline check is the cavum septi pellucidi — a small, boxy fluid space that should sit front-and-center. If it's absent, you start worrying about midline malformations. It's a tiny landmark doing a big job: present and normal is reassuring; missing is a prompt to investigate.
The heart: symmetry is your friend
The fetal heart terrifies people, and I get it — congenital heart disease is a sprawling field. But the screening logic is mercifully simple: you're checking that the building blocks are present and roughly symmetric, not naming every defect.
The cornerstone is the four-chamber view: two atria, two ventricles, the two sides about equal in size, and the crux (the central crossing point of the valves) intact. A heart that's grossly lopsided — one ventricle much smaller than the other — is the kind of thing that makes you slow down. Then you add the outflow tracts, confirming the two great vessels leave the heart and cross over each other like an X rather than running side by side.
Most serious congenital heart disease distorts either the four-chamber symmetry or the way the great vessels leave the heart. Confirm both views look normal, and you've screened out a large share of the worst lesions.
A normal four-chamber and normal outflows don't guarantee a perfect heart — some lesions hide from screening — but they catch a lot. Anything that looks off gets escalated to a dedicated fetal echocardiogram, and the bigger picture lives in adult congenital heart disease, where you see how these defects play out years later.
A bright spot in the heart — an echogenic intracardiac focus — looks alarming but is usually just a normal variant (a mineralized papillary muscle). On its own it does not mean a cardiac defect. Don't let one shiny dot hijack an otherwise normal-looking heart.
The kidneys and bladder: follow the fluid
Here's the elegant part of fetal GU imaging: the fetus's kidneys make urine, that urine becomes the amniotic fluid, and the fluid is then swallowed and recycled. So the urinary tract and the fluid around the baby are one connected system. When you understand that loop, the findings almost diagnose themselves.
The most common GU finding is hydronephrosis — dilation of the kidney's collecting system, fluid pooling where urine should be draining away. Like the brain's ventriculomegaly, mild dilation is often a benign waiting game, while severe dilation points to a true obstruction downstream. It's the same "clogged drain" story, one organ over, and it connects directly to postnatal hydronephrosis and obstruction.
A persistently enlarged bladder that won't empty suggests a blockage at the very bottom of the plumbing (a lower urinary tract obstruction). And because the bladder feeds the amniotic fluid, a severe obstruction can dry up the fluid entirely.
That brings us to the quiet hero of the whole scan:
Amniotic fluid volume is a vital sign. Too little fluid (oligohydramnios) can mean the kidneys aren't making urine or it can't get out — and severe early oligohydramnios chokes off the room the lungs need to develop. Too much fluid (polyhydramnios) can flag a swallowing or GI problem upstream. The fluid is often the first clue that something is wrong, even before you find the culprit.
How to hold all of this
You will not memorize every fetal anomaly, and nobody expects you to. What you can carry is the framework: brain = check the fluid spaces and the midline; heart = confirm symmetry and the outflows; GU = follow the urine, then check the amniotic fluid it creates. Findings get sorted into "watch and rescan" versus "send to a fetal specialist," and the trends over time — paired with growth and Doppler assessment — usually matter more than any single snapshot.
If you remember one sentence: most fetal screening is really about fluid in the wrong amount or the wrong place — in the ventricles, around the heart's symmetry, in the kidneys, and in the womb itself. Track the fluid, and you've understood most of the scan.