i hate it when this happens and i think they have a bit of a cheek tbh, i do nails only and they sit and breath in my face and cough etc for an hour, i wish they wouldnt.
however its part and parcel, much as i dont like it so i put up with it and am thankful it doesnt happen all that much. in your pregnant position, i think you have every right to try and protect yourself and agree, send texts, put a post on your facebook page and maybe a printed sign near your workstation to remind them all.
what i do do, is keep Chloraseptic throat spray (antiseptic and anaesthetic effect) and vicks first defence nasal spray in my bathroom cabinets, when someones been in with a cold/bug of some sort i use both of these as soon as they're gone, in fact i have been known to make an excuse and sneak upstairs and use it before i do their nails, lol, and after, and for the next 24 hours and hopefuly im not speaking too soon but i catch very little. i also am careful not to put my hands near my mouth and of course wash them thoroughly afterwards. i just though you might like to hear about my preventative measures in case it helps you too. hope you get well soon xx