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Submitted on: 30 May 1999.
Revised on: 23 September 1999.
Accepted on: 07 October 1999.
___________________Archives of Disease in Childhood-Fetal and Neonatal Edition
Submitted on: 12 September 2008.
Revised on: 15 November 2008.
Accepted on: 16 December 2008.
___________________Archives of Disease in Childhood-Fetal and Neonatal Edition
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