Normal Head Circumference for a 6-Week-Old Infant
For a healthy term infant at 6 weeks of age, head circumference typically ranges from 36 to 39 cm (mean approximately 37–38 cm), with males measuring slightly larger than females. 1
Measurement Standards and Interpretation
- Normal values should fall between the 3rd and 97th percentiles on sex-specific reference charts 1
- Measurements below the 3rd percentile (approximately -2 SD) define microcephaly 1
- Measurements above the 97th percentile (approximately +2 SD) define macrocephaly 1, 2
Proper Measurement Technique
To ensure accuracy, the following technique must be used:
- Use a firm, non-stretchable measuring tape positioned just above the supraorbital ridges anteriorly and around the occipital prominence posteriorly to capture the maximum circumference 1
- Apply sufficient tension to compress hair against the skull 1
- Record the measurement to the nearest 0.1 cm 1
Appropriate Growth Chart Selection
- The CDC-endorsed WHO growth charts are recommended for children aged 0–24 months in the United States 1
- At 6 weeks (approximately 1.5 months), plot the measurement on the WHO head-circumference-for-age chart specific to the infant's sex 1
Critical Clinical Context
Serial measurements are far more clinically valuable than a single measurement, as they assess growth trajectory rather than size alone 1. A single measurement at 6 weeks tells you where the infant falls on the curve, but tracking measurements over time identifies concerning patterns.
Essential Considerations:
- Measure both parents' head circumferences when interpreting infant values, as familial patterns are the most common benign cause of measurements outside typical ranges 1, 2
- Do not reassure families based solely on a single measurement without serial assessments 2
Red-Flag Findings Requiring Urgent Evaluation
- Rapidly increasing head circumference crossing percentile lines upward, especially with splaying of sagittal sutures or fontanel fullness, may indicate raised intracranial pressure and warrants urgent workup 1
- Rapid percentile crossing (e.g., from 3rd to 90th percentile) represents pathological growth requiring immediate neuroimaging 3
- Normal head growth follows consistent percentile curves, with variations of no more than 1–2 percentile lines being physiologically normal 3
Common Pitfalls to Avoid
- Do not attribute rapid percentile crossing to "catch-up growth," as this does not typically result in dramatic percentile shifts 3
- Do not dismiss macrocephaly as benign familial variant without first measuring parental head sizes and completing thorough clinical examination 2
- Do not delay imaging based on normal neurological examination alone, as some children with significant intracranial pathology may have subtle or absent neurological signs initially 3