I did this. I had DS2 in February, and returned to work in the following February and had to tell my boss I'd be leaving again in May.
It was my first day back and I hadn't intended to tell her then or there, but I had to. She was driving me to see a client and explaining that this client had been messed around by staffing changes on our side, and needed a good, consistant run with the same member of staff supporting them for the rest of the year. She was planning to introduce me on that basis. 
I decided the most professional thing to do would be to tell her there and then, but she was so shocked she nearly ran the car off the road!
I have to say, she wasn't very pleased, but 'rules is rules' and we worked for a local council, so she wasn't allowed to say anything against it.
And I got my full entitlement to maternity benefits and leave that time as well. You are entitled to it by law, no matter how short the gap!
It will be hard to tell them at work, but I'd advise that you research your position very well, make sure you are aware of your rights. Make sure you're in a union if you're not already, and give them a ring about it. I could have had a much easier time if I'd have known what was available to me in terms of taking unpaid leave, using V-time rather than going part time permanently, and taking time off for appointments.
Good luck, and...
<strong>*</strong><strong>*</strong><strong>CONGRATULATIONS!!!</strong><strong>*</strong><strong>*</strong>