I'm going to go out on a limb and assume your family was Irish Catholic. In the 1850s there were only 2 parishes in Milwaukee your ancestor could have been baptized at: St. Gall's, which then was located right in what is now downtown Milwaukee (records go back to 1849), and the Cathedral (originally St. Peter the Apostle) of St. John the Evangelist (records go back to 1840). Since technically there were no birth certificates in Milwaukee County (any prior to 1872 were filed retroactively), you need to rely on the parish records. These are available on microfilm through the LDS family history centers nationwide, as well as at some other repositories in Milwaukee. Mary Popovich Busia's Roots, Genealogy