Correction: Pessaries in multiple pregnancy as a prevention of preterm birth: the ProTwin Trial - Liem Sophie MS; Bekedam Dick J; Bloemenkamp Kitty WM; Kwee Anneke; Papatsonis Dimitri NM; van der Post Joris AM; Lim Arianne C; Scheepers Hubertina CJ; Willekes Christine; Duvekot Johannes J; Spaanderman Marc; Porath Martina; et al. (BMC, 2012)