Well, looking at your dates, I'd say it's all fairly convenient from the POV of an HoD, if you have a healthy pregnancy. Most of the face to face teaching will be done by the end of March, and you can go on ML for the Summer term. And who knows what/when/how you'll return. I'd be planning for you not to be around for the next academic year, so would be reallocating your generalist teaching & not planning on having your specialist teaching that year.
My attitude is that it'd be much the same if you had a fellowship or grant that bought you out. Except that statutory maternity pay cover is rubbish money to your employer, so mostly universities don't do much about a maternity cover, except possibly upping the part-time teaching budget. But I've always had to argue for that, so it's not a given.
Even as an HoD, I've always been really pleased for colleagues when they have a wanted pregnancy & I assume that if they tell me then it's wanted/welcome/planned (I don't tend to hear about terminations or miscarriages). It is a managerial hassle, but it's one of many, and it's usually something to celebrate. That is, unless you mess people about.
The main thing as an HoD I'd be thinking about would be covering your marking, as it looks as though covering your teaching this academic year won't be necessary - I'm being optimstic that it'll be a straightforward pregnancy & you'll manage pre-natal appointments etc around your teaching schedule. As we are so often told, pregnancy's not an illness (although when I tried that on one colleague she was extremely rude, offended, and accused me of being anti-woman & anti-family, which was a really low blow, as I was picking up her teaching on top of my workload).
So I'd talk marking options through with you, asking you what you thought we could do about covering it. There may be a couple of options:
a) someone else picks it up in May, which would require you to do a bit more during the two terms you're there to balance out the workloads;**
b) you could re-schedule your marking so it is done before you go on ML (again, assuming you pregnancy is fine - I hope it is
and you go on ML a week or so before your due date - well, that's what my colleagues have done. So you'd shift essay due dates up so you could mark before you go on leave.
c) It's trickier if it's exams, but then I think you are not going to be able to make a fuss about other people marking your courses, no matter how specialised they are.
** I know some people will say that this is unfair as when you're on ML, you're on ML and the increase in someone else's workload is not your responsibility but I tend to hope that colleagues are, well, collegial. But what would I know, as a childless person . I'm assuming you're reasonable & not so determined on your rights as to not see how your actions have an impact on other people - in general terms of course, not specifically about ML etc etc.
From an HoD POV it's all about being reasonable & flexible on both sides. One thing about that is that planning for 2016-17 will start pretty well in October 2015 - this may not be what you do, but I imagine your HoD is already thinking about 2016-17. So you need to give her/him as much notice as you reasonably can. At the 12 week point is reasonable it seems to me, or if you feel comfortable, earlier would be fine. But please don't swear your HoD to more than the usual confidentiality. S/he is going to need to plan & that may involve talking to the Dean etc about the situation.
It's all very well other posters saying that covering your ML isn't your problem. To a certain extent it's not, obviously. But only an unreasonable person would be deliberately unaware of the impact of their choices/decisions on other people's. This goes for all sorts of leave: getting a Fellowship, taking unpaid leave, sick leave for elective surgery (obviously not acute or necessary surgery etc etc). So deal openly and frankly - much in the manner you'd like to be dealt with.
Of course this is all predicated on your HoD being the paragon that I obviously am
But seriously, I've always been really pleased when colleagues have babies. I really love babies, even if I was unable to have my own. THe other stuff will get worked out, as long as you're not unreasonable in helping it to get worked out. My price for being reasonable has always been to have a good cuddle of the new born when my colleague comes in to visit!