How do we keep women after maternity leave?

The problem I work in HR at a financial services company, which boasts some very senior women who are crucial to the business and bring diversity to an otherwise male, middle-class board in a male-dominated industry. But many are choosing not to return to work after maternity leave or leave shortly afterwards. I think this is because our line managers aren’t tactful in the way they treat women before, during and after maternity leave. How can I broach this? And how can we encourage our top talent back to work so we retain our competitive edge?