Many reasons, one being infection. You could have ink reaction even if you have prior tats too. Your baby could be allergic, and you have a compromised immune system. Of course there is a very small chance of if your artist is not using sterile tools, then you could get a disease from another person such as hep C. There are other possible complications too. Most will not do it unless they do not care about the fact they could be hurting somebody. You usually have to sign a waiver saying you are not preg. Also, do not get a tat while BF.