Answer:
There is no evidence that lower back tattoos cause harm in this situation. Initiating an epidural through the ink of a lower back tattoo will not cause ink to enter the blood stream or the spinal canal, or cause further complications for the mother or baby.
Source: http://www.lifelinetomodernmedicine.com/ArticlePage.aspx?ID=302a85f8-7135-4174-bd4c-2bc4ad7e04c2&LandingID=fc6eb1da-98e4-43c7-bb9f-09c17e2a005d