First marathon! Thank feck you popped your head in - I was worried! So glad to hear everything is well.
Second, no, IQ your boss is not being fair at ALL and infact is, as summer martha says wandering into dangerous territory, legally. At this juncture, I would be going back to your doctor and explaining the situation, and how it has you crying in the toilets at work, and how stressful the situation with DH being away (and the new news that he may not be around for the birth/post natal period). Exaggerate a little if you need to (although, personally, I don't think it'll be necessary - that's more than enough stress for a person to be dealing with, let alone a pregnant one). I would be poking your boss in the eye with a sharp stick by getting myself signed off completely. Whatever your boss feels is 'fair' has to fall within the remit of the law, and whether the law is right or not, or how it makes you feel to insist your rights are protected is irrelevant. AND! at giving you rope to hang yourself with! What a fucking knob. Excuse my language but come on, does he take you for a fool?
Then. When you are signed off, make an appointment with, in the first instance CAB to find out EXACTLY what is and isn't acceptable wrt to your employment. They may be able to refer you on to someone in the legal field who specialises in that area (not suggesting you're gearing up to 'sue' them or anything, but you must know where your ground is, so that you can stand it!). I'm sure if you start a thread in the legal section (perhaps pregnancy too) you will get plenty of more specific advice from people who DO know the letter of the law and support from others who have been in your situation. The equality and human rights website is a useful jumping board too (a new equality act came into force on 1st october last year, too. Bet he's not versed in that). They can't make you take part time hours because you're struggling due to pregnancy. It is their LAWFUL duty to support you and not discriminate against you because of pregnancy. I'm seething on your behalf and hope that with a bit of virtual hand holding from us you won't take this laying down (as much as laying down probably seems attractive right now!
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And, then, when you win that battle (as you will) LET'S TAKE ON THE QUEEN! ANARCHY UK 2011!

I won't get thrown in the dungeon for treason for that, will I? I mean... Queenie's probably not a mumsnetter and I highly doubt she's expecting in May 2012 so I think I'm ok. Right? Right. Although if I vanish, ala marathon then it's possible The Firm have gotten to me.