I'm naturally dirty blonde but I used to dye my hair red.
I would have to dye it every 3-4 weeks because my hair just doesn't take color and fades horribly.
I would use one of those semi-permanent color gels/foams and mix it with my conditioner and let it sit for 3-5 minutes every few days to keep the red in.
Maybe that would help with the faded look between coloring?
Like PP said, ammonia-free/natural dyes are probably the way to go once you are pregnant, I probably would be too worried about it while TTC. From what I have read it mostly has to do with breathing in the fumes from the dye that is a concern.