Why Do Some Autistic Children Have Bigger Brains? 247social developmental milestones. Usually, at around 3 years old there is enough
evidence to suggest to parents and physicians that a child might have autism.
Interestingly, magnetic resonance imaging studies of babies that utilized
head circumference as a potential guide of brain size during the first few years
of life have identified a remarkable observation in ASD. The children with ASD
experience an unusual period of disproportionate brain growth during the first
few years of life, a period of time that coincides with the commencement
of ASD symptoms. Importantly, the rapid increase in brain size is followed by
an atypically reduced rate of brain growth. Several studies have shown that
Hind brainMid brainMid brainFore brain
Spinal cord
Cerebral hemisphereCerebellumPonsMedulla25 days35 days40 days
50 days100 days9 months5 monthsAutistic childFigure 8.4 The potential mechanism by which an autistic brain is differentiated from a
typical growing fetal brain. We propose that the actual injury occurs during the first
trimester, predominantly in the future forebrain. Certain progenitor cells are destroyed by
chemicals or other environmental insult, which allows other progenitor neurons to fill the
empty space created by the death of the target neurons. These new progenitor neurons
grow at a faster rate than the original progenitor neurons (i.e., oxytocin‐ or AVP‐receptor
positive neurons) and, subsequently, result in a larger than average brain size.