As with many questionable topics and concerns during pregnancy, there are no clear answers just cautions. The safest bet is to ask your doctor and discuss details of how often you color your hair and what kind of dye you use. Many upscale salons have discontinued using harmful chemicals and may be able to provide a safer option.
With many medical professionals, the rule of thumb is to wait until the first trimester has passed. However your OB may recommend no hair dye during any phase of your pregnancy. If this is the case and the pregnancy is going to be planned, get a professional dye job before an expected conception. Timing is key with this option.
Even if your doctor gives you the OK, you may still remain cautious. If so, consider a happy medium such as highlights. While ammonia may still be the main ingredient, highlights can be applied farther away from the scalp and less is used than with all-over color treatments, thereby lowering the amount of fumes.