The Cessation in Pregnancy Incentives Trial (CPIT): study protocol for a randomized controlled trial oleh Tappin David M, Bauld Linda, Tannahill Carol, de Caestecker Linda, Radley Andrew, McConnachie Alex, Boyd Kathleen, Briggs Andrew, Grant Liz, Cameron Alan, MacAskill Susan, Sinclair Lesley, Friel Brenda, Coleman Tim