TECHNICALITIES

Measuring Head Circumference (OFC)

As the name implies, occipitofrontal circumference is a measurement of the circumference of the head around the occiput, or posterior aspect, of the skull, to the most anterior portion of the frontal bone. The measurement should be taken with a device that cannot be stretched, such as a flexible metal tape measure. As everyone's head is slightly different, the tape should be moved around the circumference of the head in order to obtain the largest possible measurement.

It is impossible to inadvertently inflate the OFC unless a stretchable measuring device is used. Conversely, it is quite easy to obtain a measurement that is artifactually small if one does not move the tape around seeking the largest measurement. This is probably the most common reason why a child has a "small" head circumference. Remeasuring the head around the BIGGEST possible circumference may place the child within the normal range. In rare situations, such as premature closure (ossification) of the junctions (sutures) between specific skull bones, brain growth causes the skull to expand upward rather than from front to back. Children with this condition, termed craniosynostosis, may have a small measured OFC but a brain of normal size.

Does Head Circumference Really Measure Brain Growth?

In infants between 18 and 43 weeks gestation dying during the first week of life, brain weight correlated directly with head circumference using log10 transformations of the measurements (1). Postnatally, total brain DNA content (number of cells) is linearly related directly to changes in head circumference during the first six months of life, and brain weight and protein content are proportionate to cranial volume calculated from the OFC through the first year of life (2). For brain growth and head circumference beyond the first year of life, the relative rates of growth are fairly similar. Nearly all brain growth occurs in the first two years and virtually ceases by four years of age (3,4). Therefore, head circumference accurately reflects brain size and growth during gestation and the first years of life-the period of the majority of brain growth-except in unusual situations such as hydrocephalus, enlargement of the subdural space, significant scalp edema or a rickets-thickened skull.

Standards of Head Growth: What's Too Big or Too Small?

Growth curves are designed to identify children whose head circumference measures smaller or larger than the majority of children at a given age. In general, curves are constructed by taking the head circumference measurements of a representative group of children-the larger the better-and statistically determining the range wherein the majority of measurements fall. Some commonly employed growth curves use as their cutoff points for "normal growth" the 5th and 95th percentile; in other words, heads that fall outside the range of "normal" are bigger or smaller than 95% of children measured at a given age.

Most authors and all papers referenced in this paper have chosen a criterion of two standard deviations above or below the average measurement (mean) for a given age as the cutoff points for normal growth. Standard deviation is a statistical terms that is most applicable in situations where the distribution of measurements approximates a bell-shaped curve. Using two standard deviations above or below the mean encompasses a slightly larger population (3rd to 97th percentile). Therefore, more children will fall within the "normal range" when using this definition.

Does Head Circumference Vary Worldwide?

Two growth curves are commonly used for assessing head size in the United States. Head growth curves most often used during the first three years of life are based on data gathered in the United States through a study performed by the National Center for Health Statistics (NCHS) (5). The World Health Organization has adopted height and weight data from this study as optimal growth standards for an ethnically diverse, well-nourished population. The implied limits of the normal range for these curves are the 5th and 95th percentile. Therefore, some children who fall slightly above or below this range will still be in the "normal range" if this is defined as being within two standard deviations from the mean.

The Nellhaus curves (6) for children 0-18 years of age are widely used, particularly for children three years of age and older. These curves define the normal range from ± 2 standard deviations from the mean. These were developed by merging data from 14 studies of head growth worldwide published between 1948 and 1965. Nellhaus concluded that composite head circumference growth curves developed from data in these reports demonstrated the applicability of these graphs "for children regardless of skin color or geographic location." The United States medical community, solely on the basis of these data, has almost universally accepted this conclusion. However, the 15 groups of children from the 14 published articles included only two groups of African American children, from Washington, D.C., and Philadelphia, and only one group of Oriental children, from Tokyo. The remaining studies included only Caucasian children living in Scotland, Belgium, Sweden, Switzerland, England, Finland, Czechoslovakia and the United States.

With exception of the Czech Republic and Slovakia (two countries placing only a handful of children abroad), neither the Nellhaus charts nor the NCHS incorporates data from any country of the world placing children in the United States for adoption. Does this fact invalidate these curves for international adoptees? This is a difficult question to answer since specific head growth curves rarely exist from countries of interest; if they do, they may not represent the current population of children.

As a rule, both growth curves routinely employed by health-care providers in the United States represent head growth standards for a well-nourished and somewhat ethnically diverse population. They can be employed to evaluate international adoptees as long as the shortcomings of these curves are recognized. If a head growth curve is available from a child's country of origin, it may be helpful in interpreting an individual's head size in relationship to peers. Patterns of growth over time are much more informative than individual points.

Introduction | Does Head Size Correlate with Neurologic Outcome?