This is a common question, as the popularity of tattoos has been recently increasing, and in particular the lower back seems to be a very common site. Although some anesthesiologists may be reluctant to perform an epidural directly through a tattoo, most of the time this can be done safely. It is always best to ask an anesthesiologist at your hospital to discuss this concern. However, there really is no evidence that inserting a needle through a tattoo will lead to any harm. According to an opinion stated by a pathologist in the SOAP (Society for Obstetric Anesthesia and Perinatology newsletter: 


“The tattoo pigments do not stay "loose" within the dermis or connective tissue, but are rapidly assimilated by macrophages during the healing process. Many of these pigment-laden macrophages stay in the dermis, and the dermis only, for the life of the tattooee. The amount of pigment that is used in the tattoo process is quite miniscule, also. The pigments are made of inert metal salts and different structures of a phenolphthalein base, and allergic problems are nearly nonexistent. You should have no concern whatsoever in placing a needle through a tattoo and into the spinal or epidural space, as the pigment particles are "fixed" and stationary within the dermis, and cannot be mobilized by the needle or migrate along the needle track. There is really no danger at all in inserting a needle through tattooed skin.

Further reading:

http://www.soap.org/media/newsletter_summer2001.pdf 

http://www.storknet.com/cubbies/childbirth/exwc3.htm

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