YWCA
The national organization is called YWCA USA on first reference. It is headquartered in Washington, D.C. On subsequent references, use YWCA.
Be aware of that local YWCA organizations are named using YWCA followed by the location name. For example: YWCA Madison, YWCA Alaska, YWCA Boulder County, etc.
Note: For the local YWCA in Boston, Mass., organization, which is considered the birthplace of YWCA, use YWCA Boston in first reference, then YW Boston in subsequent references.