Detection of major congenital malformations depends on length of follow-up in Swedish National Health Register Data: Implications for pharmacoepidemiological research on medication safety in pregnancy.

The proportion of children with a detected MCM varied by MCM subgroup and follow-up time. In pharmacoepidemiology studies of medication safety in pregnancy using Swedish national data, the length of child follow-up should be chosen in accordance with the expected age at detection if a specific subgroup of MCM is under investigation, for example, eye and genital MCM require longer follow-up for detection than abdominal wall and digestive system MCM. However, in most circumstances, 1 year of follow-up is sufficient.