"We are thrilled to announce that Tammy is pregnant, and expecting our twins sometime around this fall," they said in a statement. "To answer the obvious question: we used an anonymous donor from a (sperm) bank."
Etheridge, 44, and Michaels, 31, held a commitment ceremony in 2003. The Grammy-winning singer has two children from her relationship with former partner Julie Cypher: daughter Bailey Jean and son Beckett. Cypher had the children through artificial insemination using a sperm donation from rock legend David Crosby.
In the announcement, posted Monday, Etheridge and Michaels said: "The twins will be a joyful addition to our loving home. We are a blessed family."
Etheridge was diagnosed with breast cancer in 2004, but is now cancer-free. She last released a greatest hits album titled, "The Road Less Traveled."
Michaels starred on the 2005 NBC comedy "Committed" and has appeared on Showtime's "The L Word."