Go for tunic dresses, look for straight or a line cuts to skim over your tummy. Darker colours will of course help and prints will help to disguise any shadows caused by a bump!
Avoid drawing attention to tummy area- belts, tucked in tops etc, dresses with waistband will draw the eye right to your middle.
Try to draw attention elsewhere- shoes/necklace -depending on what's appropriate in your workplace but drawing the eye with something away from your tummy will always help.
I write a style blog for mums (
http://cocomamastyle.com) so you should find some more useful outfit ideas on disguising tummies there which apply during early pregnancy and after!
Wishing you the very best with your pregnancy