The brain starts well before birth. Humans don't have a single on switch so much as a long bootstrap process that starts when the first few neurons start to link up and ends at death. Although most positive changes happen by ~25 years old.

What we think of as consciousness is basically the neuron's that stop focusing on what is going on and start considering options that we don't directly carry out. AKA when you actually catch a ball you don't really think about it but when you consider how you might do a better job in the future well that's consciousness. The brain is not a fixed entity but a constantly adapting system and consciousness is really best thought of as part of that adaptive process.