Month 9 and Birth

A baby is considered full term at 37 weeks gestation. The baby’s due date is at 40 weeks gestation, which is 38 weeks after conception. Most doctors will induce labor if it has not started naturally within 2 weeks of a mother’s due date.

No one knows exactly how labor begins, but research suggests that the fetus sends hormonal signals to the mother’s uterus to start labor.¹⁰ Powerful contractions push the baby out. The soft plates in his skull squeeze together and overlap to allow his large head to move through the birth canal. As far as we know, childbirth is the most stressful event of a child’s life.¹¹ Circulating stress hormones spike at about 10-15 times higher than resting levels, much higher than levels seen in severely stressed adults. Furthermore if the baby experiences oxygen deprivation during labor, the circulating stress hormones can increase to levels that would cause a stroke in an adult.¹²

The baby is born with about 40% more neurons than he will ever have again.¹⁴ Most scientists agree that humans do not grow new neurons in the cortex after birth, although premature babies may get an extra few months to finish growing neurons.¹⁵  During the next three years, these neurons form connections, called synapses, with thousands of other neurons, until there are over 1 trillion synapses in the brain.¹⁶

Development follows a pattern of blooming and pruning. Different regions of the brain show a peak number of synapses at different times. For example, the visual cortex has the maximum number of synaptic connections when the child is 2 to 4 months old, while the frontal cortex shows a peak number of synaptic connections when the child is 2 years old.¹⁸ Synapses continue to form and die off, and circuits get faster by increasing myelination until around age 25.¹⁹

Why would the brain create more neurons and synapses than it needs, only to get rid of the extras? The answer lies in the interaction of genetic and environmental factors during brain development. Basically, genes direct newly-formed neurons to their correct locations, and then sensory information and experiences support the survival and connectedness of these neurons. This allows the brain to fine-tune itself in response to external inputs. The inputs stimulate neural activity, and repeated use strengthens synapses. Synapses that do not get used very often weaken and get eliminated in the pruning process while frequently used synapses stay efficiently connected and support a child’s cognitive abilities. By blooming and pruning, both nature and nurture play vital roles in shaping a child’s brain.²⁰