A baby that is conceived during a rocky period in a marriage, when it is hoped that a baby will (even temporarily) bring the couple back together. Usually the wife plans this baby behind her husband's back and pretends to be surprised.
Lisa: I thought Rick & Joan were heading towards divorce?
Brenda: Yeah, but then Joan announced her pregnancy.
Lisa: Ooh, band-aid baby. She is too meticulous to "forget" to take her birth control.