its cos there is a 1% chance of miscarriage! my friend recently got a tattoo and found out she was preg but had a miscarriage! better 2 be safe than sorry
What lilpixie said. The risk of infection. Because a tatto piercies the skin to insert the ink, the tattoo usually will crust over (scab) for a few days. It is during this healing period when you are most susceptible to the infection. You run the risk with broken skin of infection even by the smallest things like touching the bathroom door handle after someone didn't wash their hands and then unconciously reaching up to scratch or rub your new tattoo. Sounds gross, personally better safe than sorry. Remember everything you do to yourself right now is like doing something to the baby.
the risk of infection. As safe and clean as a health certified tattoo parlor can be you always run the risk of infecton when piercing the skin (millions of times for a tattoo) Also with infection you will often get a fever which is not good for baby. I have 5 tattoos total (none during preg) and did have a reaction to the ink in one of them, its just not worth the risk to baby.