Risk of preterm birth following surgical treatment for cervical disease: executive summary of a recent symposium by Sasieni, P, Castanon, A, Landy, R, Kyrgiou, M, Kitchener, H, Quigley, M, Poon, L, Shennan, A, Hollingworth, A, Soutter, W, Freeman-Wang, T, Peebles, D, Prendiville, W, Patnick, J