The size of a baby’s head is an indicator of brain growth and overall neurological development. At birth, a newborn’s head is disproportionately large compared to the rest of the body, reflecting the brain’s rapid growth. Monitoring this size over time helps medical professionals ensure development is progressing as expected. This routine measurement allows for the early identification of potential neurological concerns.
Measuring the Baby’s Head: Prenatal and Postnatal Tracking
Medical professionals track head size before birth using specialized prenatal measurements taken during ultrasound examinations. Common measurements include the Biparietal Diameter (BPD), which measures the width of the fetal head, and the Occipitofrontal Diameter (OFD), which measures the length from front to back.
These diameters are used to calculate the Head Circumference (HC), which estimates fetal size and gestational age. Prenatal HC estimates monitor growth progression and help anticipate potential difficulties during delivery. HC is a reliable parameter for assessing fetal development throughout the second and third trimesters.
Postnatally, measuring the head circumference is a standard procedure performed at every well-child visit for the first two years of life. A flexible tape measure is placed over the most prominent part of the forehead and the widest point at the back of the head. This occipital frontal circumference (OFC) measurement is then plotted onto standardized growth charts to compare the baby’s size against established norms for their age and sex.
Normal Head Circumference and Growth Milestones
The average head circumference for a full-term newborn is approximately 34 to 35 centimeters (about 13.5 inches). This initial measurement marks the start of rapid growth. The first year accounts for the majority of head growth, with the circumference increasing by a total of around 12 centimeters.
Growth is fastest in the first three months, typically increasing by about 2 centimeters each month. This rate slows down between three and six months, generally increasing by about 1 centimeter per month. By the first birthday, head circumference is usually around 45 to 46.5 centimeters.
Measurements are evaluated by plotting them on percentile charts, which show the range of typical sizes for a population. For example, a baby on the 50th percentile has a head size larger than 50% of babies of the same age and sex. The focus is on the consistency of the growth curve, ensuring the baby tracks along their established percentile over time.
The Flexible Skull: Anatomy for Birth and Growth
The newborn skull is not a single, solid structure. It is composed of several thin, bony plates (including the frontal, parietal, and occipital bones) held together by fibrous joints called sutures. This design allows the head to navigate the birth canal and accommodate the brain’s rapid growth.
The sutures allow the bony plates to temporarily overlap and compress, a process known as molding, enabling the head to pass through the narrow birth canal. After delivery, this overlapping can leave the head with a temporary cone-like shape. Molding is a normal effect that resolves within a few days or weeks as the bones shift back into place.
Where these sutures intersect, there are wider, membrane-covered gaps known as fontanelles, or soft spots. The two most prominent are the larger, diamond-shaped anterior fontanelle at the front, and the smaller, triangular posterior fontanelle at the back. Fontanelles serve as expansion joints, allowing the skull to enlarge as the brain grows.
The posterior fontanelle is the first to close, usually fusing within the first two to three months after birth. The anterior fontanelle remains open longer to accommodate brain growth, typically closing between 7 and 18 months of age. Healthcare providers check the tension and flatness of these fontanelles, as they indicate internal pressure.
When Head Measurements Fall Outside the Norm
Deviations from the expected growth curve require further medical evaluation. Microcephaly occurs when a baby’s head circumference is significantly smaller than average, typically falling below the 3rd percentile. This condition is often associated with the underlying brain not developing or growing fully.
Conversely, macrocephaly is defined as a head circumference significantly larger than average, generally measuring above the 97th percentile. A large head size can be a harmless, inherited trait known as benign familial macrocephaly. However, it may also signal conditions like hydrocephalus (excess fluid accumulation) or megalencephaly (overgrowth of the brain structure).
A sudden, rapid acceleration or deceleration in the rate of head growth is more concerning than a consistently high or low percentile. Only a medical professional can accurately interpret these measurements by considering the child’s overall development, genetics, and neurological exam.