Lisalisa, I'm an accountant rather than a lawyer so if anything I say is irrelevant then just ignore it. In my business chargeable hours is also used as a key performance indicator and is closely monitored. I would be very surprised if your situation wasn't already being discussed by the partners in your department - it would be in my firm. However, we are (I hope) relatively sensible and sensitive to individual circumstances and if you have only been back four months then it's very early days. Did you have any discussions with your bosses before you went on maternity leave regarding arrangements when you came back?
What we do, as a partner group, on a monthly basis is to review the chargeability of all the fee earners and if anyone's utilisation is either particularly low without reason (eg holiday/illness) or consistently below target then one of us is asked to discuss with the person concerned. This is not a witch hunt or criticism - there are usually lots of good explanations but it does mean that everyone knows they can't get away with prolonged skiving (not that I'm suggesting this is any way your case so please don't take offence).
If, as a result of that discussion I and the fee earner conclude that they have capacity for more work, then we jointly come up with a set of actions to fill it. This might be as simple as letting another manager/partner know about the capacity, or might be assisting/developing a marketing project or a piece of technical research etc.etc. I usually find that with a little encouragement most people have loads of good ideas and all they need is a bit of support to have confidence to start developing them.
In your position then, I would start thinking proactively about what you could do to contribute to the future development of the business and then speak to your boss along the lines of "I was thinking that given I have some time available at present it would be a good idea to.....". This would enable you to gauge their level of support for your idea, hopefully enlist some practical help if needed, and also provide you with a defence in the event that they tried to pull you up on lack of chargeable hours as you would then be able to justify the time you had spent as being preagreed with Mr/Ms X.
In my experience people who take ownership of problems like this do distinguish themselves and this can often help at bonus/promotion time.
I appreciate that this approach might not work in your firm or be relevant in your line of business. You have my sympathy - I hate not being busy. If I can help in any way please let me know.