AC stands for abdominal circumference, a measurement of the distance around your baby’s belly taken during a routine ultrasound. It’s one of the most important numbers your provider tracks because the size of the fetal abdomen reflects how well your baby is growing, particularly whether they’re getting enough nutrition. AC is measured in millimeters and compared against standardized growth charts to see where your baby falls relative to others at the same gestational age.
How AC Is Measured
During an ultrasound, the sonographer captures a cross-sectional image of your baby’s abdomen at a very specific level. They’re looking for two key landmarks: the stomach (visible as a dark bubble on the left side) and a J-shaped structure in the middle of the abdomen where the umbilical vein branches into the portal vein. That J-shape should sit about one-third of the way across the belly.
The image needs to show a circular cross-section, not an oval one. If the kidneys are visible, or if the umbilical cord insertion point shows up, the angle is wrong and the sonographer will reposition. These details matter because even small errors in the image plane can throw off the measurement. Once the correct view is captured, the sonographer traces or plots the outline of the abdomen to calculate the circumference.
Why AC Matters More Than Other Measurements
Your ultrasound report likely includes several measurements: biparietal diameter (the width of the head), head circumference, femur length, and AC. Of these, abdominal circumference is the most sensitive indicator of how your baby is growing day to day. The reason is straightforward: the baby’s abdomen houses the liver, which is the primary storage site for glycogen (the body’s short-term energy reserve). When a baby isn’t getting enough nutrients through the placenta, the liver shrinks first, and the AC drops before other measurements change. Conversely, when a baby is getting excess glucose, the liver and surrounding fat deposits enlarge, pushing the AC up.
All four measurements feed into a formula (commonly the Hadlock formula) that calculates your baby’s estimated fetal weight. That estimate has an accepted margin of error of about 10% in either direction. The AC measurement itself has been shown to have a mean error of around 5%, and when different sonographers measure the same baby, their results can vary by up to about 9%. This is why a single measurement that’s slightly off isn’t cause for alarm. Providers look at trends across multiple ultrasounds.
Typical AC Values by Gestational Age
AC grows steadily throughout pregnancy. Based on NICHD growth standards, here are approximate values at three common milestones:
- 20 weeks: The 50th percentile is about 152 mm, with the 10th percentile around 141 mm and the 90th around 164 mm.
- 28 weeks: The 50th percentile is about 242 mm, with the 10th percentile around 227 mm and the 90th around 258 mm.
- 36 weeks: The 50th percentile is about 329 mm, with the 10th percentile around 306 mm and the 90th around 354 mm.
Your baby doesn’t need to be at the 50th percentile to be healthy. What matters is that the measurement falls within a normal range and tracks consistently along a growth curve over time. A baby who measures at the 25th percentile throughout pregnancy is growing normally. A baby who drops from the 60th to the 15th percentile over several weeks is the one who needs closer attention.
What a Small AC Can Mean
When the AC (or the estimated fetal weight calculated from it) falls below the 10th percentile for gestational age, providers consider the possibility of fetal growth restriction. The Society for Maternal-Fetal Medicine defines growth restriction as an estimated fetal weight or AC below the 10th percentile. This doesn’t automatically mean something is wrong. Some babies are constitutionally small, meaning they’re genetically programmed to be on the smaller side, and they’re perfectly healthy.
The distinction between a small-but-healthy baby and one that isn’t growing properly comes down to follow-up testing. When growth restriction is suspected, the next step is typically Doppler ultrasound of the umbilical artery, which measures blood flow between the placenta and the baby. Normal blood flow is reassuring. Abnormal flow patterns suggest the placenta isn’t delivering enough oxygen and nutrients, and monitoring becomes more frequent, sometimes weekly or even multiple times per week depending on the severity. Babies with severely restricted growth (below the 3rd percentile) are watched especially closely.
In cases where the AC is small but the head circumference remains normal, providers call this asymmetric growth restriction. It typically points to a placental problem that developed later in pregnancy, since the baby’s body redirects blood flow to protect the brain at the expense of abdominal growth.
What a Large AC Can Mean
A larger-than-expected AC often raises questions about macrosomia, the term for a baby that’s significantly larger than average (generally over 4,000 or 4,500 grams at birth). Research has found that an AC of 350 mm or greater near the end of pregnancy predicts macrosomia with about 77% accuracy, and carries a positive predictive value of roughly 93%. In one study, the median AC in babies born macrosomic was about 368 mm, compared to 330 mm in those who were not.
A large AC is particularly relevant in pregnancies complicated by gestational diabetes. Glucose crosses the placenta freely, but maternal insulin does not. When a mother’s blood sugar runs high, the baby receives that excess glucose and responds by producing more of its own insulin starting as early as 11 weeks. Insulin acts as a growth hormone in the fetus, stimulating fat deposition and organ enlargement, especially in the abdomen. This is why AC is often the first measurement to increase in pregnancies with poorly controlled blood sugar, even before the overall weight estimate looks concerning.
A large AC near term also helps providers assess the risk of shoulder dystocia, a delivery complication where the baby’s shoulders get stuck after the head is delivered. Studies have found that babies who experienced shoulder dystocia consistently had an AC above 350 mm, regardless of their actual birth weight.
What Happens After an Abnormal AC
If your baby’s AC falls outside the expected range, your provider will likely schedule more frequent ultrasounds to track growth over time. For a small AC, the standard approach includes umbilical artery Doppler assessments every one to two weeks. If blood flow patterns worsen, monitoring increases to weekly or even two to three times per week. Heart rate monitoring (cardiotocography) is typically added after the baby reaches viability, usually weekly for straightforward cases and more often if Doppler findings are concerning.
For a large AC, the follow-up depends on the suspected cause. If gestational diabetes is involved, tighter blood sugar management often slows the accelerated growth. Your provider may repeat growth ultrasounds every two to four weeks to see whether the trajectory is changing. In both scenarios, the timing and method of delivery may be adjusted based on how the baby responds over the following weeks.
A single AC measurement that looks high or low is not a diagnosis on its own. The built-in margin of error in ultrasound, the way the baby is positioned, and even how full the baby’s stomach is at the moment can all affect the number. Trends over time give a much clearer picture than any single data point.