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The initial sample size calculation in our protocol (Hegeman et al, BMC Pregnancy Childbirth, 2009, 9:44) was based on the expected proportion of 'bad neonatal outcome' in the intervention group (3.9%) and control group (7.2%) and accounts for the fact that the outcomes in children form multiple pregnancies are non-independent using an intra class correlation of 0.6. As the intervention is performed on the mother, analysis should be done on the maternal level. This adjustment was made during recruitment and approved by the medical ethics committee of the Academic Medical Centre in Amsterdam (ref. No. MEC 09/107). The sample size is calculated based on the primary outcome 'bad neonatal outcome'.

Erratum to: Pessaries in multiple pregnancy as a prevention of preterm birth: the ProTwin Trial