31.1 Introduction: Infant, Child, and Adolescent Development
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Women usually detect fetal movements 16 to 20 weeks into
the pregnancy; the fetus can be artificially set into total body
motion by in utero stimulation of its ventral skin surfaces by the
14
th
week. The fetus may be able to hear by the 18
th
week, and it
responds to loud noises with muscle contractions, movements,
and an increased heart rate. Bright light flashed on the abdomi-
nal wall of the 20-week pregnant woman causes changes in fetal
heart rate and position. The retinal structures begin to function
at that time. Eyelids open at 7 months. Smell and taste are also
developed at this time, and the fetus responds to substances that
may be injected into the amniotic sac, such as contrast medium.
Some reflexes present at birth exist in utero: the grasp reflex,
which appears at 17 weeks; the Moro (startle) reflex, which
appears at 25 weeks; and the sucking reflex, which appears at
about 28 weeks.
Nervous System.
The nervous system arises from the neu-
ral plate, which is a dorsal ectodermal thickening that appears
on about day 16 of gestation. By the sixth week, part of the
neural tube becomes the cerebral vesicle, which later becomes
the cerebral hemispheres (Fig. 31.1-2).
The cerebral cortex begins to develop by the 10
th
week, but
layers do not appear until the sixth month of pregnancy; the sen-
sory cortex and the motor cortex are formed before the asso-
ciation cortex. Some brain function has been detected in utero
by fetal encephalographic responses to sound. The human brain
weighs about 350 g at birth and 1,450 g at full adult develop-
ment, a fourfold increase, mainly in the neocortex. This increase
is almost entirely because of the growth in the number and
branching of dendrites establishing new connections. After birth,
the number of new neurons is negligible. Uterine contractions
can contribute to fetal neural development by causing the devel-
oping neural network to receive and transmit sensory impulses.
Pruning
Pruning refers to the programmed elimination during develop-
ment of neurons, synapses, axons, and other brain structures
from the original number, present at birth, to a lesser number.
Thus, the developing brain contains structures and cellular ele-
ments that are absent in the older brain. The fetal brain gener-
ates more neurons than it will need for adult life. For example,
in the visual cortex, neurons increase in number from birth to
3 years of age, at which point they diminish in number. Another
example is that the adult brain contains fewer neural connec-
tions than were present during the early and middle years of
childhood. Approximately twice as many synapses are present
in certain parts of the cerebral cortex during early postnatal life
than during adulthood.
Pruning occurs to rid the nervous system of cells that
have served their function in the development of the brain.
Some neurons, for example, exist to produce neurotrophic or
growth factors and are programmed to die—a process called
apoptosis—when that function is fulfilled.
The implication of these observations is that the immature
brain can be vulnerable in locations that lack sensitivity to
injury later on. The developing white matter of the human brain
before 32 weeks of gestation is especially sensitive to damage
from hypoxic and ischemic injury and metabolic insults. Neu-
rotransmitter receptors located on synaptic terminals are subject
to injury from excessive stimulation by excitatory amino acids,
(e.g., glutamate, aspartate), a process referred to as excitotoxic-
ity. Research is proceeding on the implications of such events in
the etiology of child and adult neuropsychiatric disorders such
as schizophrenia.
Maternal Stress
Maternal stress correlates with high levels of stress hormones
(epinephrine, norepinephrine, and adrenocorticotropic hor-
mone) in the fetal bloodstream, which act directly on the fetal
neuronal network to increase blood pressure, heart rate, and
activity level. Mothers with high levels of anxiety are more
likely to have babies who are hyperactive, irritable, and of low
birth weight, and who have problems feeding and sleeping
than are mothers with low anxiety levels. A fever in the mother
causes the fetus’s temperature to rise.
A
B
Figure 31.1-1
A.
Sonogram of fetus at 9 weeks.
B.
Same fetus at 15 weeks. (Courtesy K.C. Attwell, M.D.)
