Neither Sex at Birth nor Current Gender Identity map exactly to Administrative Gender. For most patients, obviously, these are the same things, but once transgender issues come into play, they may be different. Administrative Gender is what the patient officially is - typically for billing/reporting purposes. Typically, this lags behind the current gender identity, and is not fluid like gender identity can be, though it may eventually be reconciled, depending on culture and jurisdication. Sex at Birth is different thing again.
Typically, current gender identity is an observation, with LOINC code like 76691-5 (Gender Identity). You can use an observation Sex at Birth (say 76689-9 "Sex assigned at birth", though note that's not quite the same thing). If you're in USA, see http://hl7.org/fhir/us/core/StructureDefinition-us-core-birthsex.html