Ethics Statement
This study was approved by the institutional review board of Asan Medical Center (IRB No. 2008-0616), Samsung Medical Center (IRB No. 2009-02-021), Severance Hospital (IRB No. 4-2008-0588), and CHA Medical Center (IRB No. 2010- 010). Written informed consent was confirmed by each IRB and obtained from the parents of each infant.