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Head Circumference (HC) in Pregnancy: Measurement and Normal Range
Oct 26, 2026
HC in pregnancy is one of the key measurements taken during an ultrasound scan. It records the distance around your baby's head. Along with other readings, HC helps your doctor track growth and development. Understanding what HC means in pregnancy can make your scan reports much less confusing. This blog explains what HC is, how it is measured, and the normal range by week.
These figures are indicative averages and vary between growth charts. Your doctor uses the chart your hospital follows.
Key Takeaways
- HC (head circumference) is the ultrasound measurement around your baby's head, used to track growth.
- It is measured from the second trimester and rises from about 73 mm at 13 weeks to about 350 mm at 40 weeks.
- One reading above or below average is usually normal; doctors judge the trend across scans and other measurements.
- Genetics, dating accuracy, baby's position and maternal health can all shift an HC reading.
Table of Contents
- What Does HC Mean in a Pregnancy Ultrasound?
- How Is Head Circumference Measured?
- Normal Range for Head Circumference By Week of Pregnancy
- What If HC Is Above or Below the Normal Range?
- Factors That Affect Fetal Head Circumference
- Conclusion
- FAQs
What Does HC Mean in a Pregnancy Ultrasound?
So what is HC in ultrasound reports? HC stands for head circumference, the measurement around your baby's head. It is one of four core growth readings, alongside the abdomen, femur, and head width from side to side. Together these estimate your baby's size and weight. In pregnancy, HC simply shows whether the head is growing at a healthy rate. Doctors plot the value on a growth chart and compare it with the average for that week. A reading near the average is reassuring.How Is Head Circumference Measured?
Doctors measure head circumference during a routine ultrasound, usually from the second trimester. The sonographer takes a clear cross-section image of the baby's head at a set level. A measuring tool then traces an oval around the outer edge of the skull. The machine calculates the distance in millimetres automatically. The reading is quick, painless, and part of your normal anomaly and growth scans. Because the baby moves and its position varies, small differences between scans are normal. Your gynaecologist interprets the HC measurement alongside other fetal growth measurements to assess your baby’s development more accurately.Normal Range for Head Circumference By Week of Pregnancy
The normal range for fetal head circumference rises steadily as your baby grows. The table below shows approximate average HC values by week. Your scan chart may differ slightly, so use this only as a general guide for HC in pregnancy.| Week of pregnancy | Approx. average HC |
|---|---|
| 13 weeks | 73 mm |
| 16 weeks | 124 mm |
| 20 weeks | 175 mm |
| 24 weeks | 224 mm |
| 28 weeks | 265 mm |
| 32 weeks | 296 mm |
| 36 weeks | 326 mm |
| 40 weeks | 350 mm |
What If HC Is Above or Below the Normal Range?
A reading outside the average does not usually signal a problem. Many healthy babies measure a little above or below the line. A single HC measurement that is higher or lower than expected does not necessarily indicate a problem. Your doctor will check your pregnancy dates and compare the measurement with other growth indicators. If the HC is smaller than expected, a follow-up scan may be recommended to monitor your baby’s growth. A larger HC may reflect normal variation or family traits. If the measurements continue to fall outside the expected range or other concerns are present, your doctor may recommend a consultation with a fetal medicine specialist for further evaluation. Try not to read too much into one number for HC in pregnancy.Factors That Affect Fetal Head Circumference
Several normal factors influence how HC measures on a given scan.- Genetics, as head size often runs in families.
- The accuracy of your pregnancy dating in early scans.
- The baby's position and movement during the scan.
- Maternal health, including nutrition and conditions such as diabetes.
Conclusion
HC in pregnancy is a helpful marker of your baby's healthy growth. It is read alongside other measurements, not alone. A reading near the average is reassuring, and small differences are normal. Discuss any questions about your scan report with your gynaecologist for clear advice.Medical Disclaimer
This blog is for general information only. It is not a substitute for medical advice. Please consult a qualified gynaecologist for guidance suited to your pregnancy.Frequently asked questions
Does a large HC mean I will need a C-section?
Not on its own. A large HC can mean a bigger baby, but delivery depends on many factors. Your doctor decides based on your full assessment near term.
How accurate is HC measurement on ultrasound?
HC is one of the more reliable growth measurements. A small margin of error is normal, so doctors track the trend across scans rather than one reading.
Can HC size predict a baby's intelligence?
No. Head circumference reflects growth, not intelligence. A reading above or below average says nothing about your child's mental development.
Why does my baby's HC differ from the BPD reading?
HC measures around the head, while BPD measures across it. They can differ because of head shape and position, which is completely normal.
How fast does head circumference increase?
HC grows steadily through pregnancy, by roughly 10 to 14 mm a week in the second trimester. Growth slows gradually in the final weeks.
How often is HC measured during pregnancy?
HC is usually measured at the anomaly scan and at later growth scans. Your doctor may add scans if closer monitoring is needed."
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