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.