I don't think CIC actually counts an unborn child as a dependent. As long as you land before the birth of the child, then I cannot see any problems.
When you marry your girlfriend and start sponsorship, the marriage and birth will be after you are given your PR, so all will be perfectly legal. There is no misrepresentation as far as I can see, unless there is something else you are not telling us.
Besides, rejessome said there is no problem and she knows what she is talking about, as she is a highly respected immigration consultant on this forum.