Table 3.
Characteristics of study participants from the Longitudinal Survey of Newborns in the 21st Century, 2001–2009 (girls n = 7,229).
| Only children | Age gap shorter than 1.5 years | Age gap 1.5 years to shorter than 4 years | Age gap 4–8 years | ||
|---|---|---|---|---|---|
| Variable | No (%) or mean (SD) | No (%) or mean (SD) | No (%) or mean (SD) | No (%) or mean (SD) | |
| Number of participants | 2,133 (29.5) | 330 (4.6) | 3,950 (54.6) | 816 (11.3) | |
| Overweighta | Yes | 329 (15.4) | 47 (14.2) | 441 (11.2) | 97 (11.9) |
| No | 1,804 (84.6) | 283 (85.8) | 3,509 (88.8) | 719 (88.1) | |
| Household income at age 0.5, million JPY | Mean, SD | 3.9 (2.4) | 3.6 (2.1) | 3.8 (2.1) | 3.9 (3.9) |
| Birth weight | <2,500 g | 223 (10.5) | 26 (7.9) | 329 (8.3) | 81 (9.9) |
| ≧2,500 g | 1,910 (89.6) | 304 (92.1) | 3,621 (91.7) | 735 (90.1) | |
| Mother's age at birth, years | Mean, SD | 30.7 (4.8) | 27.9 (4.3) | 28.4 (3.6) | 28.5 (3.7) |
| <25 | 241 (11.3) | 80 (24.2) | 647 (16.4) | 133 (16.3) | |
| 25–29 | 729 (34.2) | 156 (47.3) | 2,092 (53.0) | 409 (50.1) | |
| 30–34 | 744 (34.9) | 74 (22.4) | 1,054 (26.7) | 238 (29.2) | |
| 35–39 | 359 (16.8) | 18 (5.5) | 152 (3.9) | 36 (4.4) | |
| ≧s40 | 60 (2.8) | 2 (0.6) | 5 (0.1) | 0 (0.0) | |
| Mother's education | Junior high school | 68 (3.2) | 14 (4.2) | 82 (2.1) | 10 (1.2) |
| High school | 761 (35.7) | 132 (40.0) | 1,300 (32.9) | 244 (29.9) | |
| Vocational school | 853 (40.0) | 139 (42.1) | 1,821 (46.1) | 414 (50.7) | |
| Higher education | 400 (18.8) | 39 (11.8) | 696 (17.6) | 144 (17.7) | |
| Others | 2 (0.1) | 2 (0.6) | 3 (0.1) | 0 (0.0) | |
| Missing | 49 (2.3) | 4 (1.2) | 48 (1.2) | 4 (0.5) | |
| Mother's smoking status at age 0.5 | No | 1,792 (84.0) | 274 (83.0) | 3,465 (87.7) | 696 (85.3) |
| Yes | 331 (15.5) | 54 (16.4) | 476 (12.1) | 117 (14.3) | |
| Missing | 10 (0.5) | 2 (0.6) | 9 (0.2) | 3 (0.4) | |
| Father residence at age 0.5 | Yes | 2,021 (94.8) | 326 (98.8) | 3,916 (99.1) | 803 (98.4) |
| No | 112 (5.3) | 4 (1.2) | 34 (0.9) | 13 (1.6) | |
| Gestational period, weeks | 22–36 | 94 (4.4) | 11 (3.3) | 102 (2.6) | 22 (2.7) |
| 37–41 | 2,010 (94.2) | 315 (95.5) | 3,802 (96.3) | 778 (95.3) | |
| ≧42 | 29 (1.4) | 4 (1.2) | 46 (1.2) | 16 (2.0) | |
| Duration of breastfeeding | None | 187 (8.8) | 18 (5.5) | 158 (4.0) | 24 (2.9) |
| 1–2 months | 462 (21.7) | 103 (31.2) | 564 (14.3) | 141 (17.3) | |
| 3–5 months | 470 (22.0) | 108 (32.7) | 801 (20.3) | 178 (21.8) | |
| 6 or more months | 999 (46.8) | 98 (29.7) | 2,382 (60.3) | 460 (56.4) | |
| Missing | 15 (0.7) | 3 (0.9) | 45 (1.1) | 13 (1.6) | |
| Rapid growth during early childhoodb | No | 1,096 (51.4) | 161 (48.8) | 2,157 (54.6) | 460 (56.4) |
| Yes | 836 (39.2) | 127 (38.5) | 1,525 (38.6) | 299 (36.6) | |
| Missing | 201 (9.4) | 42 (12.7) | 268 (6.8) | 57 (7.0) | |
| Exercise habit at age 8 | Yes | 1,064 (49.9) | 134 (40.6) | 1,947 (49.3) | 417 (51.1) |
| No | 1,069 (50.1) | 196 (59.4) | 2,003 (50.7) | 399 (48.9) | |
| Breakfast consumption at age 8 | Yes | 2,020 (94.7) | 316 (95.8) | 3,786 (95.9) | 775 (95.0) |
| No | 59 (2.8) | 11 (3.3) | 94 (2.4) | 29 (3.6) | |
| Missing | 54 (2.5) | 3 (0.9) | 70 (1.8) | 12 (1.5) | |
| Excessive screen time at age 8c | No | 673 (31.6) | 120 (36.4) | 1,389 (35.2) | 231 (28.3) |
| Yes | 1,456 (68.3) | 210 (63.6) | 2,556 (64.7) | 584 (71.6) | |
| Missing | 4 (0.2) | 0 (0.0) | 5 (0.1) | 1 (0.1) | |
| Irregular bedtime at age 8 | No | 2,065 (96.8) | 320 (97.0) | 3,878 (98.2) | 791 (96.9) |
| Yes | 64 (3.0) | 10 (3.0) | 68 (1.7) | 24 (2.9) | |
| Missing | 4 (0.2) | 0 (0.0) | 4 (0.1) | 1 (0.1) | |
| Healthy dinner habits at age 8d | Yes | 611 (28.7) | 104 (31.5) | 1,353 (34.3) | 283 (34.7) |
| No | 1,515 (71.0) | 224 (67.9) | 2,576 (65.2) | 529 (64.8) | |
| Missing | 7 (0.3) | 2 (0.6) | 21 (0.5) | 4 (0.5) | |
| Grandparent cohabitation at age 8 | No | 1,592 (74.6) | 260 (78.8) | 3,123 (79.1) | 660 (80.9) |
| Yes | 541 (25.4) | 70 (21.2) | 827 (20.9) | 156 (19.1) | |
| BMI at birth | Mean, SD | 12.5 (1.1) | 12.5 (1.2) | 12.5 (1.1) | 12.5 (1.1) |
| Unavailable | 25 (1.2) | 3 (0.9) | 26 (0.7) | 13 (1.6) | |
| BMI at age 1.5 | Mean, SD | 16.1 (1.4) | 16.2 (1.4) | 16.1 (1.3) | 16.1 (1.4) |
| Unavailable | 218 (10.2) | 51 (15.5) | 324 (8.2) | 70 (8.6) | |
| Father's age at first birth, years | Mean, SD | 32.9 (6.0) | 30.2 (4.7) | 30.2 (4.7) | 30.1 (4.4) |
| <25 | 160 (7.5) | 51 (15.5) | 410 (10.4) | 83 (10.2) | |
| 25–29 | 527 (24.7) | 130 (39.4) | 1,667 (42.2) | 339 (41.5) | |
| 30–34 | 675 (31.7) | 86 (26.1) | 1,279 (32.4) | 285 (34.9) | |
| 35–39 | 458 (21.5) | 43 (13.0) | 449 (11.4) | 91 (11.2) | |
| ≧40 | 244 (11.4) | 16 (4.9) | 125 (3.2) | 13 (1.6) | |
| Missing | 69 (3.2) | 4 (1.2) | 20 (0.5) | 5 (0.6) | |
| Father's education | Junior high school | 114 (5.3) | 30 (9.1) | 182 (4.6) | 34 (4.2) |
| High school | 718 (33.7) | 142 (43.0) | 1,440 (36.5) | 275 (33.7) | |
| Vocational school | 345 (16.2) | 55 (16.7) | 689 (17.4) | 151 (18.5) | |
| Higher education | 845 (39.6) | 96 (29.1) | 1,579 (40.0) | 350 (42.9) | |
| Others | 2 (0.1) | 0 (0.0) | 1 (0.03) | 1 (0.1) | |
| Missing | 109 (5.1) | 7 (2.1) | 59 (1.5) | 5 (0.6) | |
| Father's smoking status at age 0.5 | No | 816 (38.3) | 102 (30.9) | 1,626 (41.2) | 336 (41.2) |
| Yes | 1,226 (57.5) | 224 (67.9) | 2,295 (58.1) | 467 (57.2) | |
| Missing | 91 (4.3) | 4 (1.2) | 29 (0.7) | 13 (1.6) | |
| Residential area | 20 designated cities | 587 (27.5) | 60 (18.2) | 861 (21.8) | 187 (22.9) |
| Other cities | 1,214 (56.9) | 189 (57.3) | 2,370 (60.0) | 505 (61.9) | |
| Rural | 332 (15.6) | 81 (24.6) | 719 (18.2) | 124 (15.2) |
SD, standard deviation.
a
Overweight was determined as BMI z-score 1 SD or more according to the WHO criteria at the age of 8 years old (22).
b
Rapid growth during infancy was determined as gaining more than 0.67 SD of weight from age 0.5 years to age 1.5 years (32).
c
Excessive screen time was determined as 2 or more h of watching TV or playing computer games on either weekdays or weekends at the age of 8 years old (33).
d
Unhealthy dinner habits were determined by frequent or occasional consumption of instant food and precooked food, using food delivery, or eating out on weekdays, which was asked at the age of 8 years old.