Methods
In MoBa, 78,117 mothers of 91,378 children completed a series of questionnaires in the 17th and 30thweek of gestation of each unique pregnancy (Figure 1). The questionnaires asked the mothers to report their anxiety level, in addition to providing information regarding their age, education, marital status, smoking and drinking habits, and their relationship with their partner. The MoBa cohort12 was also linked to the Medical Birth Registry of Norway (MBRN), which contains detailed medical information about new-born children (gender, birthweight, and gestational age) as well as their mothers (parity and birth complications).
Among these full data sets, there were 12,480 pairs of siblings. The mean age difference between siblings was 2.77 years. The numbers of pairs dropped slightly after excluding half-siblings who had the same mother but different fathers (Figure 1). We used version eight of the quality-assured data files.