Supported by grants (HL098554 and HL098354) from the NHLBI, by grants (HD21410, HD27915, HD27917, HD27869, HD34116, HD34208, HD40485, HD40500, HD40512, HD40544, HD40545, HD40560, HD53097, HD53118, HD68268, HD68258, HD68282, and HD36801) from the NICHD, and by a grant (UL1 TR000040) from the National Center for Advancing Translational Sciences, National Institutes of Health. Disclosure forms provided by the authors are available with the full text of this article at NEJM.org.