|Embryo at 4 weeks|
Electrical brain activity is first detected between the 5th and 6th week of gestation, though this is still considered primitive neural activity rather than the beginning of conscious thought, something that develops much later in fetation. Synapses begin forming at 17 weeks, and at about week 28 begin to multiply at a rapid pace which continues until 3–4 months after birth. It is not until week 23 that the fetus can survive, albeit with major medical support, outside of the womb, because it does not possess a sustainable human brain until that time.
|Embryo at 4 weeks after fertilization|
|Fetus at 8 weeks after fertilization|
|Fetus at 18 weeks after fertilization|