10 Things to Look for During Your 20-Week Anatomy Scan

The 20-Week Scan: Your Most Important Ultrasound

Around weeks 18–22, you'll be scheduled for what's officially known as the fetal anomaly scan—though most parents call it the "big" scan, the "half-way scan," or simply "the anatomy scan." It's usually the longest and most detailed appointment of your entire pregnancy, lasting 30–60 minutes, and for very good reason: this is when a trained sonographer methodically examines virtually every part of your developing baby's body.

While you might be primarily focused on finding out the sex (if you've chosen not to know already), the sonographer is running through a detailed checklist of developmental markers. Understanding what they're looking for will help you feel less anxious, ask better questions, and understand any follow-up recommendations.

The Anatomy Scan Checklist

1. The Brain and Neural Development

The brain is one of the first structures assessed. Your sonographer will measure the head circumference (HC) and biparietal diameter (BPD—the width of the skull) to confirm appropriate growth. They'll also look for specific structures including the cerebellum (involved in balance and coordination), the lateral ventricles (fluid-filled chambers), and the cavum septum pellucidum, a thin membrane that should be present in the first two trimesters. Abnormalities in these structures can be associated with conditions like ventriculomegaly or, in severe cases, agenesis of the corpus callosum—though these findings always require specialist review.

2. The Face and Lips

The sonographer will examine the profile view of the face and look directly at the lips to screen for a cleft lip or palate. In 2D ultrasound this can be tricky to visualise, but 3D probes make cleft lip much more obvious. They'll also check for the nasal bone—its absence can sometimes be associated with chromosomal anomalies. A profile view also checks for the positioning of the ears and the overall symmetry of the face.

3. The Heart

Cardiac assessment is often described as the most technically challenging part of the scan. Your sonographer needs to clearly visualise four heart chambers of approximately equal size, plus the outflow tracts—the major vessels (aorta and pulmonary artery) leaving the heart. Baby's position is crucial here; if they're lying awkwardly, the sonographer may need you to walk around for a few minutes or return for a second appointment. Heart abnormalities are the most common type of fetal anomaly, affecting around 8 in every 1,000 births.

4. The Spine

A careful scan up and down the length of the spine checks that all vertebrae are properly formed, aligned, and covered by skin. This is the primary screening for spina bifida—a neural tube defect where the spinal canal doesn't fully close. The scan also looks at the curvature of the spine (identifying potential scoliosis) and confirms the skin surface is intact.

5. The Kidneys and Bladder

Both kidneys should be visible, sitting symmetrically either side of the spine, and measuring within normal size range. The bladder should appear as a dark (fluid-filled) oval structure in the pelvis. The fact that baby fills and empties their bladder while in the womb (they're swallowing amniotic fluid and urinating it back out) is actually a great sign the entire urinary system is functioning. Mild kidney pelvis dilation (pyelectasis) is common and usually resolves, but is worth monitoring.

6. The Stomach

The stomach should appear as a dark bubble on the left side of the abdomen. Its presence confirms that baby is swallowing amniotic fluid correctly. If the stomach bubble is absent or right-sided, this can prompt further investigation.

7. The Abdominal Wall

The abdominal wall should be fully closed around the umbilical cord insertion point. The sonographer checks for conditions like gastroschisis (where bowel herniates through a gap in the abdominal wall) or omphalocele (where organs protrude through the umbilical opening). Both conditions are detectable by ultrasound and allow medical teams to plan specialist care in advance of birth.

8. The Limbs and Hands

All four limbs are measured and assessed for appropriate length. The femur length (FL) is measured as a growth marker. Hands and feet are examined for the correct number of digits, though counting all 10 fingers and toes precisely is notoriously difficult in ultrasound—curled fists are the bane of every 20-week sonographer! The sonographer will try to confirm no major abnormalities such as clubfoot or limb shortening.

9. The Placenta and Umbilical Cord

The position of the placenta matters enormously. A low-lying placenta (covering or near the cervical opening) can indicate placenta previa—a condition that affects delivery planning. Most low-lying placentas at 20 weeks will migrate upward as the uterus grows (this is normal), so a follow-up scan at 32–34 weeks is usually recommended. The umbilical cord is also checked: it should have three vessels (two arteries and one vein). A two-vessel cord can be a normal variation but is worth investigating.

10. Amniotic Fluid Levels

The amount of amniotic fluid surrounding baby is measured using the amniotic fluid index (AFI). Too little fluid (oligohydramnios) or too much (polyhydramnios) can indicate kidney or swallowing problems respectively, or sometimes issues with the placenta. Normal fluid levels are a reassuring sign of a healthy, functioning system.

What if They Ask You to Come Back?

Try not to panic. It is extremely common to be asked to return for a repeat or supplemental scan. Baby may be lying in a position that prevents clear visualisation of the heart outflow tract or the spine. "Incomplete scan" does not mean "problem found"—it means "we didn't get the view we needed." A follow-up is just a second attempt to tick the boxes.

If a genuine concern is identified, you'll be referred to a fetal medicine specialist (a perinatologist) for a more detailed Level 2 or 3 ultrasound and, where appropriate, genetic testing such as amniocentesis. Finding something at 20 weeks gives medical teams time to prepare, advise, and in many cases, plan treatment that begins immediately at birth.

Can You Find Out the Sex at the 20-Week Scan?

Yes! The genitalia are well-developed by 18–20 weeks and clearly visible in most positions. In Australia and the UK, sonographers will tell you the sex if you ask; in some other countries this varies. Keep in mind: accuracy sits around 95–99%, with the very small remaining percentage due to cord being mistaken for a penis, or a girl being viewed at an unfortunate angle. If in doubt—especially if told "probably a girl"—ask the sonographer how confident they are.

If you're impatient and can't wait until 20 weeks, check out our AI Baby Portrait to see a photorealistic prediction of your baby's face from your current ultrasound scan.