Exposure variables
MSD was calculated using the data of questionnaire in M-T2.24 The participants were categorized into five groups according to MSD during pregnancy: G1 (MSD < 6.0 h), G2 (MSD 6.0–7.9 h), G3 (MSD 8.0–8.9 h), G4 (MSD 9.0–9.9 h), and G5 (MSD 10.0–12.0 h) based on the report of Ministry of Health, Labour and Welfare, Japan,25 which suggests 6.0–7.9 h of sleep as the appropriate sleep duration for general adults to prevent several health problems. The participants who reported MSD >12.0 h were excluded from the present study because these participants were considered to have reported inaccurate MSD.