I hate to sound like a reactionary.  I have been in the field for a very long 
time and in my 30+ years I have come to respect exposure to chemicals.  And I 
worked the whole time during both pregnancies (many years ago).  Knowing what 
I know today, thanks to chemical hygiene and MSDS, I would have to do 
something else during those months.  Xylene is a mutagen and formalin a 
carcinogen.  Hematoxylin is a carcinogen.  We used to "bath" in these 
reagents (gloves were not provided then.. too expensive).  And too, what if 
something happens to her baby?  What are the possibilities of litigation?  
Are gloves, aprons and masks really enough protection?  You have a tough 
question and I really don't know what the answer should be.  Presently, I do 
not know of any regs that prohibit pregnant women from working in a 
laboratory setting.   But we all  have choices in life and your tech knowing 
the possible dangers of the chemicals  can make her choice.