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Open Access Correction

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

Sophie MS Liem15*, Dick J Bekedam1, Kitty WM Bloemenkamp2, Anneke Kwee3, Dimitri NM Papatsonis4, Joris AM van der Post5, Arianne C Lim5, Hubertina CJ Scheepers6, Christine Willekes6, Johannes J Duvekot7, Marc Spaanderman8, Martina Porath9, Jim van Eyck10, Monique C Haak11, Marielle G van Pampus12, Hein W Bruinse3, Ben Willem J Mol5 and Maud A Hegeman1

Author Affiliations

1 Department of Obstetrics and Gynaecology, Onze Lieve Vrouwe Gasthuis Amsterdam, Amsterdam, the Netherlands

2 Department of Obstetrics and Gynaecology, Leiden University Medical Center, Leiden, the Netherlands

3 Department of Obstetrics and Gynaecology, University Medical Centre, Utrecht, the Netherlands

4 Department of Obstetrics and Gynaecology, AMPHIA hospital, Breda, the Netherlands

5 Department of Obstetrics and Gynaecology, Academic Medical Centre Amsterdam, Amsterdam, the Netherlands

6 Department of Obstetrics and Gynaecology, Academic Hospital Maastricht, Maastricht, the Netherlands

7 Department of Obstetrics and Gynaecology, Erasmus Medical Center Rotterdam, Rotterdam, the Netherlands

8 Department of Obstetrics and Gynaecology, University Medical Center St Radboud Nijmegen, Nijmegen, the Netherlands

9 Department of Obstetrics and Gynaecology, Máxima Medical Center Veldhoven, Veldhoven, the Netherlands

10 Department of Obstetrics and Gynaecology, Isala Hospital, Zwolle, the Netherlands

11 Department of Obstetrics and Gynaecology, VU Medical Center Amsterdam, Amsterdam, the Netherlands

12 Department of Obstetrics and Gynaecology, University Medical Center Groningen, Groningen, the Netherlands

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BMC Pregnancy and Childbirth 2012, 12:37  doi:10.1186/1471-2393-12-37

Published: 23 May 2012

First paragraph (this article has no abstract)

The initial sample size calculation in our protocol [1] 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 from 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. The sample size is calculated based on the primary outcome 'bad neonatal outcome'. In the control group, 'bad neonatal outcome' is expected in 7.2% of the children <a onClick="popup('http://www.biomedcentral.com/1471-2393/12/37/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1471-2393/12/37/mathml/M1">View MathML</a>. In this calculation, the first rate represents the probability that a patient delivers at that gestational age, whereas the second rate represents the probability of 'bad neonatal outcome' at that particular gestational age. In case of treatment, 'bad neonatal outcome' is then expected in 3.9% of the children <a onClick="popup('http://www.biomedcentral.com/1471-2393/12/37/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1471-2393/12/37/mathml/M2">View MathML</a>. On the mother level this corresponds to an expected ‘bad neonatal outcome’ in at least one of two children of 12.4% in the control group and 6.7% in case of treatment. Using a two-sided test with an alpha of 0.05 and a power of 0.80 we need 400 women in the control group and 400 in the intervention group.