Table 1

Baseline characteristics of children by breastfeeding groups in the longitudinal survey of babies in the 21st century (nbaseline=43 383)

 Exclusively formula fed (n=2597)Partially breast fedExclusively breast fed (n=9521)
 1–2 M (n=8592)3–5 M (n=8625)6–7 M (n=14 048)
Biological factor
Birth weight, g30643059307530883111
 1 (no older siblings)49%49%52%54%41%
Maternal age at delivery
Socioeconomic factor
Maternal educational attainment*
 High school or less63%58%47%37%37%
 Two-year college or vocational school31%34%41%45%45%
 Four-year college or higher6%8%12%18%18%
Maternal smoking status*
 Light smoker17%18%14%8%6%
 Heavy smoker14%14%7%2%2%
Marital status
 Not married2%2%1%1%1%
Family income*522511554594576
 In 10 000 yen (SD)(558)(318)(391)(366)(353)
Region of birth
  • *There were 3030 missing cases for maternal educational attainment, 281 missing cases for maternal smoking status and 2767 missing cases for family income.