There is no brain function in you, but babies have a functioning brain by the end of the first trimester
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During the first trimester, the brain develops rapidly and makes up nearly half of the fetus’s weight. For comparison, by the time your baby is born, the brain is only 10 percent of their body weight. In the first trimester, the brain will grow millions of neurons, which connect across synapses to direct movement and growth.
The communication between neurons is what helps the fetus learn to move, although during the first trimester you probably won’t feel any movement.
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https://flo.health/pregnancy/pregnancy-health/fetal-development/fetal-brain-development
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