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| Embryo at 4 weeks |
One way to observe prenatal development is via ultrasound images. Modern 3D ultrasound images provide greater detail for prenatal diagnosis than the older 2D ultrasound technology. While 3D is popular with parents desiring a prenatal photograph as a keepsake, there are no definitive studies linking ultrasound to any adverse medical effects. The following 3D ultrasound images were taken at different stages of pregnancy.
At the beginning of the fetal stage, the risk of miscarriage decreases sharply, all major structures including the head, brain, hands, feet, and other organs are present, and they continue to grow and develop. When the fetal stage commences, a fetus is typically about 30 mm (1.2 inches) in length, and the heart can be seen beating via sonograph, the fetus bends the head, and also makes general movements and startles that involve the whole body. Some fingerprint formation occurs from the beginning of the fetal stage.
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.
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| Embryo at 4 weeks after fertilization |
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| Fetus at 8 weeks after fertilization |
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| Fetus at 18 weeks after fertilization |
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