Table 2 Characteristics of the participants
| Â | Total | Boy | Girl | |||
|---|---|---|---|---|---|---|
n = 27,870 | n = 14,429 | n = 13,441 | ||||
No. or mean | % or SD | No. or mean | % or SD | No. or mean | % or SD | |
Gestational age (week) | ||||||
  < 37 | 1277 | 4.6 | 764 | 5.3 | 513 | 3.8 |
 37–41 | 26,357 | 94.6 | 13,542 | 93.9 | 12,815 | 95.3 |
  ≥ 42 | 236 | 0.8 | 123 | 0.9 | 113 | 0.8 |
Birth weight (g) | ||||||
  ≥ 2500 | 25,616 | 91.9 | 13,384 | 92.8 | 12,232 | 91.0 |
  < 2500 | 2254 | 8.1 | 1045 | 7.2 | 1209 | 9.0 |
 Paternal age at child birth (years old) | 31.7 | 5.3 | 31.8 | 5.3 | 31.7 | 5.3 |
  < 25 | 1822 | 6.5 | 922 | 6.4 | 900 | 6.7 |
 25–29 | 8060 | 28.9 | 4151 | 28.8 | 3909 | 29.1 |
 30–34 | 10,236 | 36.7 | 5332 | 37.0 | 4904 | 36.5 |
 35–39 | 5583 | 20.0 | 2887 | 20.0 | 2696 | 20.1 |
 40+ | 2105 | 7.6 | 1100 | 7.6 | 1005 | 7.5 |
 unknown | 64 | 0.2 | 37 | 0.3 | 27 | 0.2 |
 Maternal age at child birth (years old) | 29.7 | 4.2 | 29.7 | 4.2 | 29.7 | 4.2 |
  < 25 | 2843 | 10.2 | 1465 | 10.2 | 1378 | 10.3 |
 25–29 | 11,059 | 39.7 | 5747 | 39.8 | 5312 | 39.5 |
 30–34 | 10,414 | 37.4 | 5402 | 37.4 | 5012 | 37.3 |
 35–39 | 3204 | 11.5 | 1629 | 11.3 | 1575 | 11.7 |
  ≥ 40 | 350 | 1.3 | 186 | 1.3 | 164 | 1.2 |
Paternal education | ||||||
 less than high school | 1936 | 6.9 | 1019 | 7.1 | 917 | 6.8 |
 high school | 10,773 | 38.7 | 5588 | 38.7 | 5185 | 38.6 |
 some college | 4461 | 16.0 | 2308 | 16.0 | 2153 | 16.0 |
 college or higher | 10,700 | 38.4 | 5514 | 38.2 | 5186 | 38.6 |
Maternal education | ||||||
 less than high school | 1076 | 3.9 | 539 | 3.7 | 537 | 4.0 |
 high school | 10,470 | 37.6 | 5414 | 37.5 | 5056 | 37.6 |
 some college | 12,074 | 43.3 | 6330 | 43.9 | 5744 | 42.7 |
 college or higher | 4250 | 15.2 | 2146 | 14.9 | 2104 | 15.7 |
Number of siblings | ||||||
 none | 12,772 | 45.8 | 6488 | 45.0 | 6284 | 46.8 |
 1 | 11,048 | 39.6 | 5841 | 40.5 | 5207 | 38.7 |
  ≥ 2 | 4050 | 14.5 | 2100 | 14.6 | 1950 | 14.5 |
 Living with grandparent(s) (answer = yes) | 5866 | 21.0 | 3065 | 21.2 | 2801 | 20.8 |
Annual household income (JPY million) | ||||||
  <  4 | 7966 | 28.6 | 4150 | 28.8 | 3816 | 28.4 |
 4–5.9 | 10,811 | 38.8 | 5576 | 38.6 | 5235 | 38.9 |
 6–7.9 | 5544 | 19.9 | 2845 | 19.7 | 2699 | 20.1 |
 8–9.9 | 2099 | 7.5 | 1127 | 7.8 | 972 | 7.2 |
  ≥ 10 | 1450 | 5.2 | 731 | 5.1 | 719 | 5.3 |
Multiple birth | ||||||
 singleton | 27,314 | 98.0 | 14,129 | 97.9 | 13,185 | 98.1 |
 twin | 546 | 2.0 | 295 | 2.0 | 251 | 1.9 |
 triplet | 10 | 0.04 | 5 | 0.03 | 5 | 0.03 |
 History of hospital visits for child’s congenital diseases (answer = yes) | 624 | 2.2 | 344 | 2.4 | 280 | 2.1 |
 History of hospital admissions for child’s congenital diseases (answer = yes) | 267 | 1.0 | 164 | 1.1 | 103 | 0.8 |
Child’s temperament (answer = yes) | ||||||
 active | 14,737 | 52.9 | 7405 | 51.3 | 7332 | 54.5 |
 shy of strangers | 2891 | 10.4 | 1181 | 8.2 | 1710 | 12.7 |
 playful | 10,360 | 37.2 | 6182 | 42.8 | 4178 | 31.1 |
 short temper | 4551 | 16.3 | 2445 | 16.9 | 2106 | 15.7 |
 careful | 4172 | 15.0 | 2323 | 16.1 | 1849 | 13.8 |
 aggressive | 12,169 | 43.7 | 5254 | 36.4 | 6915 | 51.4 |
 timid | 2084 | 7.5 | 1476 | 10.2 | 608 | 4.5 |
 obedient | 6513 | 23.4 | 3603 | 25.0 | 2910 | 21.7 |
 competitive | 8094 | 29.0 | 3700 | 25.6 | 4394 | 32.7 |
 independent | 12,378 | 44.4 | 5465 | 37.9 | 6913 | 51.4 |
 stubborn | 3994 | 14.3 | 2361 | 16.4 | 1633 | 12.1 |
 full of curiosity | 11,305 | 40.6 | 5967 | 41.4 | 5338 | 39.7 |
 fickle | 4346 | 15.6 | 2164 | 15.0 | 2182 | 16.2 |
 restless | 6318 | 22.7 | 3845 | 26.6 | 2473 | 18.4 |
 shy | 9400 | 33.7 | 4663 | 32.3 | 4737 | 35.2 |
 spoilt | 15,849 | 56.9 | 9150 | 63.4 | 6699 | 49.8 |
 easygoing | 2142 | 7.7 | 1214 | 8.4 | 928 | 6.9 |
 impatient | 1622 | 5.8 | 845 | 5.9 | 777 | 5.8 |