Synaptic pruning or axon pruning is the process of synapse elimination that occurs between early childhood and the onset of puberty in many mammals. Pruning starts near the time of birth and is completed by the time of sexual maturation in humans. At birth, the human brain consists of approximately 86 (± 8) billion neurons. The infant brain will increase in size by a factor of up to 5 by adulthood. Two factors contribute to this growth: the growth of synaptic connections between neurons, and the myelination of nerve fibers; the total number of neurons, however, remains the same. Pruning is influenced by environmental factors and is widely thought to represent learning. After adolescence, the volume of the synaptic connections decreases again due to synaptic pruning.