The 20-week anatomy scan, explained
A detailed head-to-toe ultrasound around the halfway point of pregnancy — and the most important imaging study of the entire 40 weeks.
Written and reviewed by the babybumpkit editorial team.
What the anatomy scan is
The anatomy scan — also called the level-2 ultrasound, mid-pregnancy ultrasound, or fetal-survey scan — is a detailed ultrasound exam that systematically reviews your baby's anatomy from head to toe. It's the most detailed ultrasound of pregnancy and is offered as a routine part of prenatal care in most countries.
Unlike the brief 'is everything OK' scans earlier in pregnancy, this one is a structured examination. The sonographer follows a checklist of anatomical structures, measuring some and visually confirming others, and produces a written report for your provider.
It's also the scan where most parents who want to know biological sex find out — though that's a side benefit, not the primary purpose.
When it's scheduled
The anatomy scan is typically performed between 18 and 22 weeks of gestation, most often around 20 weeks. Earlier than 18 weeks, structures aren't yet developed enough to see clearly; later than 22 weeks, the baby is more cramped and some views become harder.
Some clinics schedule it slightly earlier (around 18 weeks) or slightly later (around 22 weeks) based on their imaging protocols. The window is flexible; the result is similar.
Plan for about an hour at the clinic. The scan itself takes 30–45 minutes; you'll spend additional time on check-in, gel cleanup, and (often) discussing results with the sonographer.
What's measured and checked
The sonographer takes a series of measurements to confirm baby's growth is on track: head circumference, biparietal diameter (across the head), abdominal circumference, and femur length. These are plotted against gestational-age norms.
They visually check major organ systems: brain structures (lateral ventricles, cerebellum, cisterna magna), face (palate visualization, lip continuity), heart (four-chamber view, outflow tracts, valves), lungs, diaphragm, stomach, kidneys and bladder, spine (full length, normal curvature), abdominal wall, and all four limbs with hands and feet.
They also assess the environment around the baby: placenta location (anterior, posterior, low-lying), amniotic fluid volume, and umbilical-cord vessels (normal is two arteries and one vein).
Biological sex is examined if you've consented. Accuracy is high (>95%) when baby cooperates and the sonographer can get a clear view between the legs. If the position doesn't allow a confident call, you'll typically be offered a follow-up scan.
What the scan can and can't detect
The anatomy scan can detect many — but not all — structural abnormalities. It's very good at picking up: significant heart defects, spina bifida and other neural tube defects, anencephaly, gastroschisis and omphalocele (abdominal wall defects), cleft lip (cleft palate alone is harder), major limb anomalies, kidney anomalies, and severe growth restriction.
It's less reliable at picking up: subtle heart defects, very small structural variations that emerge later in pregnancy, or anything that depends on tissue function rather than appearance (such as how a kidney works).
What it can't detect: most chromosomal conditions (the conditions don't have a visual signature that ultrasound shows reliably) — those need NIPT, CVS, or amnio. Single-gene disorders. Autism, ADHD, or other neurodevelopmental conditions. Birth weight (that's not knowable yet).
If the scan finds something
Most anatomy scans (around 95%) come back completely normal. If something is identified, your provider will discuss what was found, what additional information would help (often a follow-up ultrasound with a maternal-fetal medicine specialist), and what your options are.
Findings range from minor (a small kidney variant that resolves on its own) to significant (a complex heart defect that may need surgery after birth). The path forward depends entirely on what was found — there's no single 'something was found' protocol.
Genetic counselors and maternal-fetal medicine (MFM) specialists are available for support and second opinions on findings. Many findings turn out to be either normal variants or conditions that can be managed; some lead to harder decisions. Your medical team will walk you through specifics.
3D and 4D ultrasounds
3D and 4D ultrasounds (the kind that produce facial 'photos' and short videos) are usually elective add-ons, not part of the standard medical anatomy scan. They're offered by both medical clinics (sometimes during the 20-week visit) and standalone 'keepsake' ultrasound centers.
From a medical standpoint, 3D/4D adds little diagnostic value beyond what 2D imaging provides. From a parent standpoint, it's a personal choice — some find the images meaningful, others find them uncanny. Standalone keepsake-only ultrasound centers aren't medically supervised; the FDA recommends against using them in place of medical ultrasound.
Frequently asked questions
Sources and medical references
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