Data were from the Swedish National Board of Health and Welfare and Statistics Sweden, from 1991 to 2008. Linkage between registries was performed using the unique personal identification number issued to Swedish residents. The Swedish Medical Birth Register (SMBR) contains data on more than 98% of all births in Sweden from 1973 onwards. Starting with the first visit at the end of the first trimester, information on all hospital births is gathered prospectively and includes demographic data, reproductive history and complications during pregnancy, delivery and the neonatal period. The SMBR was validated in 2002 and the quality is generally high.