Normal Head Circumference for a 6-Week-Old Infant
For a healthy term 6-week-old infant, the normal head circumference typically ranges from approximately 36 to 39 cm, with a mean around 37-38 cm, though this varies by sex (males slightly larger) and should be plotted on WHO growth charts between the 3rd and 97th percentiles. 1, 2
Measurement Standards and Technique
Head circumference should be measured using a firm, non-stretchable measuring tape placed just above the supraorbital ridges anteriorly and around the occipital prominence posteriorly to obtain the maximum circumference. 1, 2
The tape must have sufficient tension to compress the hair against the skull, and measurements should be recorded to the nearest 0.1 cm. 1, 2
The infant should be held with the head upright during measurement to ensure accuracy. 2
Growth Chart Selection and Interpretation
The WHO growth charts are recommended for children aged 0-24 months in the United States, as they represent optimal growth patterns under ideal conditions and include head circumference curves up to 60 months of age. 1
At 6 weeks of age (approximately 1.5 months corrected), you should plot the measurement on the WHO head circumference-for-age chart specific to the infant's sex. 1
Normal measurements fall between the 3rd and 97th percentiles (or between -2 and +2 standard deviations from the mean). 1, 2
Clinical Context and Serial Measurements
A single measurement reflects size, whereas serial measurements are essential for assessing growth trajectory, which is far more clinically valuable than any isolated value. 1, 2
Head circumference below the 3rd percentile (< -2 SD) indicates microcephaly, while measurements above the 97th percentile (> +2 SD) indicate macrocephaly, both warranting further evaluation. 2, 3
Important Considerations for Preterm Infants
If the infant was born preterm, age must be corrected for gestational age when plotting on growth charts until approximately 18-24 months. 4, 5
For infants born at very low birth weight (≤1000 g), head circumference may track below reference curves even with normal neurodevelopment, with mean differences of approximately 1.6 cm at 18 months compared to term infants. 5
Parental Factors
Parental head circumferences should be measured when interpreting an infant's measurements, as familial patterns are the most common benign cause of measurements outside typical ranges. 1, 3
Maternal height and weight influence newborn head circumference by up to 1.2 cm between extreme maternal stature groups, though this effect is less pronounced than for birth length. 6
Red Flags Requiring Further Evaluation
Head circumference more than 4 standard deviations above the mean mandates comprehensive diagnostic workup regardless of parental measurements. 3
Disproportionate head growth relative to length may indicate intrauterine growth restriction, congenital infections, or genetic disorders and requires investigation. 2
Rapidly increasing head circumference crossing percentile lines upward, especially with splaying of sagittal sutures or fontanel fullness, may indicate increased intracranial pressure and requires urgent evaluation. 1