I have not experienced such a situation in peron, but I have seen one of my colleagues come back fom a maternity leave at top tier consultancy where we used to work together.
I was amazed at the amount of hours she was working directly after coming back, so for sure she did not decrease her billability. What she did do, though, is that she got involved in more local projects, so that she decreased her travel time. In this way, she could still perform at top level, but she still managed to save travel time to be able to spend it with her baby. Maybe this was her trick?
But, the truth is also that her husband took the paternal leave to take care of the baby in this period, so she was greatly supported in the process.
Good luck with your re-adaptation,