My advice would be to concentrate firstly on the existing clients, to try to make sure you don't lose any. Make sure you communicate with them, tell them when you're leaving (my hairdresser didn't, so I missed out) and when you're coming back, and tell them that you are coming back to reassure them.
Start collecting their email addresses - and contact details. Email them a month before you go on leave to fit in their last-minute appointments; email them to tell them about the baby - and your expected return date - make it about them, not about you; and email them when you're back again.
Then think about gaining some new clients to make up for any possible shortfall. Prepare a postcard or a flyer (Vistaprint is cheap if you want to design your own; moo.com do some lovely little mini cards you could put round). Spend a little bit of money and have it delivered to local addresses. there are companies that deliver for as little as 2p or 3p per flyer - or just do it yourself at the weekends - a few hundred at a time. If you include some form of offer on the flyer, you will get abetter response. Perhaps something like £5 off a cut/blow dry or something??
One thing to consider might be that you work one or two days for the first few weeks back. Could you manage to schedule people in in advance to fit your likely timings? So that you have a fully booked diary for when you come back?
The more you can do in the next few months to build up a solid, loyal following, the better. You may well lose a few clients once you've had the baby, but if you've built up your client base to more than it was anyway, then you should be OK.
Good luck.