I've swung in my opinion a few times after reading the OP. I think YABU for reasons I will outline in a minute but maybe IABU to think that?
it's a toughie!
Ok so, first of all, smellyfishead, I am most definitely NOT bitter about any previous experiences as I only have one dc and that is by choice. Just wanted to put that out there before I get jumped on as obviously being bitter about something.
From a work POV I think you are BU. Nobody is indispensible (?sic) or irreplaceable and only providing your services to your employers for about 20% of your entire employment means you are putting your position in a precarious state. You may not care, fair play if you don't.
There are laws that prevent discrimination against women who become pregnant but that doesn't mean they have to be flogged to within an inch of their existence (there's an ancient law that states pregnant women can piss in a policemans hat if she is caught short, doesn't mean I would have done it when I was pregnant!). It is attitudes and piss taking like this that, I feel, makes it very difficult to be taken seriously in the world of employment. Of course it's wrong that women of child bearing age are deemed less employable because they may fall pregnant, but they could end up with an employee like this who they still have to pay whilst on maternity, find a temporary replacement and pay them as well then have to find yet another replacement 4 months after employee returns again.
And will your insides not be a bit of a mess having 3 children in 3 years?
Good luck with keeping your job if you continue to go ahead with this, I feel you may need it (I have a friend who works for the local council and she says they have all kinds of loopholes to get rid of 'troublesome' employees. Not sure how true that is though)