HC/AC on an ultrasound refers to the ratio between your baby’s head circumference (HC) and abdominal circumference (AC). These are two of the most important measurements taken during a prenatal ultrasound, and comparing them helps assess whether your baby is growing proportionally. A typical HC/AC ratio changes throughout pregnancy, starting above 1.0 in the second trimester (the head is larger than the belly) and gradually approaching 1.0 as the due date nears.
What HC and AC Actually Measure
Head circumference is the distance around your baby’s skull, measured on a cross-sectional image that cuts through two specific brain structures: the thalami (a relay center deep in the brain) and a small fluid-filled space between the brain’s hemispheres. The sonographer traces an ellipse around the outer edge of the skull on this view.
Abdominal circumference is the distance around your baby’s belly at a precise level. The sonographer looks for a cross-section that shows the stomach (visible on the left side) and a J-shaped structure in the middle of the abdomen where the umbilical vein branches into a blood vessel in the liver. A good AC image should look circular, not oval, and the kidneys should not be visible. Getting this slice exactly right matters because the AC is the single most sensitive measurement for detecting growth problems.
Why the Ratio Matters
Each measurement on its own tells you something about size. The ratio between them tells you something about proportion. A baby whose head and belly are growing at expected rates will have an HC/AC ratio that falls within a predictable range for each gestational age. When the ratio drifts outside that range, it can signal that one part of the body is growing faster or slower than the other.
The HC/AC ratio is also one of several measurements plugged into formulas that estimate your baby’s weight. The most widely used formulas, developed by Hadlock in the 1980s, combine HC, AC, and femur length (FL) to predict birth weight. A formula using just HC and AC has a margin of error around 9%, while adding the femur length brings it down to about 7.6%. For all formulas, the accepted accuracy window is plus or minus 15% of actual birth weight, and accuracy tends to drop for very large babies.
High HC/AC Ratio and Growth Restriction
When the HC/AC ratio is higher than expected, the baby’s head is relatively large compared to its belly. This pattern is the hallmark of what’s called asymmetric fetal growth restriction (FGR), which accounts for roughly 70% to 80% of all growth restriction cases. It typically develops in the late second or third trimester and is most often caused by problems with the placenta not delivering enough nutrients.
The reason the head stays relatively normal while the belly falls behind is a survival mechanism called “brain sparing.” When a baby isn’t getting enough blood flow, its body redirects circulation to protect the brain. The result is that brain growth continues at a more normal pace, but organs in the abdomen (especially the liver, which stores energy) shrink. On ultrasound, this shows up as a head that measures closer to normal and an abdomen that measures small, pushing the HC/AC ratio upward.
If your provider suspects growth restriction based on this ratio, they’ll typically follow up with Doppler ultrasound, which measures blood flow through the umbilical cord and the baby’s brain arteries. These blood flow patterns give a clearer picture of how the baby is coping and help guide decisions about monitoring or timing of delivery.
Low HC/AC Ratio and Macrosomia
A lower-than-expected HC/AC ratio means the belly is large relative to the head. This pattern is commonly seen in babies of mothers with diabetes, both gestational and pre-existing. High blood sugar crosses the placenta and causes the baby to produce extra insulin, which acts as a growth hormone. The excess insulin drives fat accumulation and organ enlargement in the trunk, while the head grows at a more typical rate.
Research in Obstetrics & Gynecology Science found that an HC/AC ratio below 0.95 was significantly more common in mothers with diabetes than in healthy mothers. That same cutoff was also linked to a higher risk of the baby developing low blood sugar (neonatal hypoglycemia) after birth. The association was strongest in mothers with pre-existing diabetes, where the odds of neonatal hypoglycemia were dramatically elevated when the ratio dropped below 0.95.
For providers managing diabetic pregnancies, this ratio serves as one early signal that the baby may be disproportionately large, which can influence decisions about blood sugar management, delivery planning, and newborn monitoring.
How Providers Use the Ratio Today
The HC/AC ratio was historically used to classify growth restriction as either “symmetric” (everything small, often linked to chromosomal issues) or “asymmetric” (head spared, belly small, often linked to placental problems). Current guidelines from the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommend moving away from those labels. The reason: the pattern doesn’t reliably predict the underlying cause. Chromosomal problems can cause asymmetric growth, and placental insufficiency can cause symmetric growth.
Instead of relying on the HC/AC ratio alone, modern practice diagnoses growth restriction by combining a small AC or estimated fetal weight (below the 10th percentile) with additional findings. Depending on gestational age, these include Doppler measurements of blood flow in the uterine arteries, umbilical cord, and fetal brain arteries, or a significant drop in growth percentile between serial scans.
This doesn’t mean the HC/AC ratio is obsolete. It remains a useful screening tool, a quick flag that something may be off with proportional growth. But it’s now one piece of a larger puzzle rather than a standalone diagnostic criterion. If you see HC/AC on your ultrasound report, it’s there because your provider is tracking whether your baby’s head and body are growing in step with each other, one of the most basic and informative checks in prenatal care.