Fig. 1

Selection process for participant eligibility in Hokkaido Study. The prospective cohort study was performed between July 2002 and October 2005 at Toho Hospital (Hokkaido Study on Environment and Children’s Health). A total of 1,796 pregnant women were invited to participate in the study during a routine gynaecological checkup and 514 women agreed. Women who were registered for the Cord Blood Bank were not eligible to participate. Assessment of depression during pregnancy was conducted between October 2002 and April 2004; 309 women were involved in the assessment and completed questionnaires (60 % of the initial cohort). Pregnant women who were outside of the study period or who failed to complete the assessment were eliminated from the cohort. Postnatal depression was assessed in 267 mothers between 1 and 4 months post-delivery, and infant development was assessed in 154 mother–infant pairs during the period from 5 months and 16 days to 6 months and 15 days after birth (50 % follow-up rate). Excluded participants were those who did not complete the protocol due to miscarriage, stillbirth, multiple birth, relocation, death of the infant or voluntary withdrawal from the study. Statistical analysis was conducted for 154 mother–infant pairs. EPDS Edinburgh Postnatal Depression Scale, EES evaluation of Environmental Stimulation, BSID-II Bayley Scales of Infant Development II