Ethical approval
The study was approved by the Institutional Review Boards of Asan Medical Center (IRB No. 2008-0616), the Samsung Medical Center (IRB No. 2009-02-021), the Severance Hospital (IRB No. 4-2008-0588), and the CHA Medical Center (IRB No. 2010-010). Written informed consent was obtained from each mother and her husband before the study-related interview was conducted. The obtainment of consent was confirmed by the IRB.