Dear Gemma Cleary
Humans can do lots of things with their brains that other animals can't do nearly as well. 'Thinking' is one of them.
To do these things the brain has to be the right size and it has to be very complicated inside. Also it must not have finished growing before the human is born, otherwise it would not be able to learn like humans learn.
Other animals are born already having most of the knowlege that they will need in life. That knowledge is called instinct.
Of course they can learn new things as well. But probably they can't think like we do...and imagine new things, and so on.
So when the baby is growing inside its mother a great deal of energy has to go into making sure the brain grows big enough. There's plenty of time after the baby has been born for the rest of its body to catch up. After all, it doesn't need big strong legs at first because it doesn't need to walk. It is better to be weak and defenceless and close to its parents. Then it will quickly learn all the special things that humans learn when they are close to other humans--like language, for instance.
Also, if the baby's brain was bigger at birth then its head would be bigger so the mother's hips would have to be wider. Well, there's a limit to how wide hips can grow!