Why does food dye that has been identified as a chemical mutagen poses as a greater danger for a developing fetus than for an adult?

Chemical Mutagens - Chemical mutagens can be divided into two classes those mutagens which can cause mutations to both replicating and non replicating DNA, are called as class I mutagens, those chemicals, which affect replicating nucleic acids are called as class II mutagens. So, a fetus is replicating DNA alot more than we are so im assuming that there is a higher risk for cell mutations in a fetus than a grown person :S not sure....i have to answer the same question as well. right now. BAH!