just as a general rule for headaches...
drink lots of water...you will need more because you are in an airconditioned environmet AND cos you are pg
ask your boss for a screen sheild (cant remember their actual name) but threy sit over your screen and take away the glare, cheap as too
take regular breaks

And I would get my eyes tested. A lot of headaches can be caused by eye strain, so if I start gettigng headaches for no reason I always go and check my script.

HTH...and GL with the pregnancy!