One approach is to export the expression to MathML. An example:
In[1]:= myEq=(x+y)/z;
In[2]:= ExportString[myEq,"MathML"]
Out[2]= <math xmlns='http://www.w3.org/1998/Math/MathML'>
<mfrac>
<mrow>
<mi>x</mi>
<mo>+</mo>
<mi>y</mi>
</mrow>
<mi>z</mi>
</mfrac>
</math>
Copy the output and paste into a Word document. Word should automatically format it for you.