Table 1.
Demographic and obstetric characteristics of the infants
| Characteristic | n (%) or mean (SD) |
|---|---|
| Month of birth, (n = 81,615) | |
| January | 6,400 (7.8%) |
| February | 5,736 (7.0%) |
| March | 6,175 (7.6%) |
| April | 6,308 (7.7%) |
| May | 6,559 (8.0%) |
| June | 6,292 (7.7%) |
| July | 7,171 (8.8%) |
| August | 8,190 (10.0%) |
| September | 8,305 (10.2%) |
| October | 7,691 (9.4%) |
| November | 6,461 (7.9%) |
| December | 6,327 (7.8%) |
| Season of birth, (n = 81,615) | |
| Spring (Apr–Jun) | 19,159 (23.5%) |
| Summer (Jul–Sep) | 23,666 (29.0%) |
| Autumn (Oct–Dec) | 20,479 (25.1%) |
| Winter (Jan–Mar) | 18,311 (22.4%) |
| Maternal age (years), (n = 81,611) | |
| < 25 | 7,105 (8.7%) |
| 25–29 | 22,273 (27.3%) |
| 30–34 | 29,405 (36.0%) |
| 35–39 | 18,963 (23.2%) |
| ≥ 40 | 3,865 (4.7%) |
| Annual household income (JPY), (n = 75,701) | |
| < 4 million | 29,578 (39.1%) |
| 4 – < 6 million | 25,337 (33.5%) |
| ≥ 6 million | 20,786 (27.5%) |
| Highest maternal educational level, (n = 80,778) | |
| < 13 | 27,846 (34.5%) |
| 13 – 15 | 34,581 (42.8%) |
| ≥ 16 | 18,351 (22.7%) |
| Maternal history of allergic disease, (n = 81,203) | |
| Yes | 40,799 (50.2%) |
| Maternal intake of vitamin D during pregnancy (μg), (n = 81,177) | |
| Mean (SD) | 4.3 (2.8) |
| Maternal active smoking during pregnancy, (n = 80,587) | |
| Never-smokers | 47,828 (59.4%) |
| Quit smoking before pregnancy | 19,133 (23.7%) |
| Quit smoking during early pregnancy | 10,563 (13.1) |
| Continued smoking | 3,063 (3.8%) |
| Maternal passive smoking during pregnancy, (n = 80,979) | |
| Almost never | 51,302 (63.4%) |
| Less than 1 day per week | 9,712 (12.0%) |
| 2–3 days per week | 6,500 (8.0%) |
| 4–6 days per week | 3,877 (4.8%) |
| Every day | 9,588 (11.8%) |
| Infant sex, (n = 81,613) | |
| Male | 41,903 (51.4%) |
| Female | 39,710 (48.7%) |
| Gestational weeks (w), (n = 81,459) | |
| Mean (SD) | 38.9 (1.5) |
| Feeding method during 1 month after birth, (n = 81,350) | |
| Breastfeeding only | 34,548 (42.5%) |
| Mixed feeding | 45,740 (56.2%) |
| Infant formula only | 1,062 (1.3%) |
| Presence of older siblings at 1 month after birth, (n = 81,615) | |
| Yes | 42,695 (52.3%) |
| Pet ownership, (n = 80,830) | |
| Yes | 18,290 (22.6%) |
| Registered regional center, (n = 81,615) | |
| Hokkaido | 6,505 (8.0%) |
| Miyagi | 6,700 (8.2%) |
| Fukushima | 11,145 (13.7%) |
| Chiba | 4,720 (5.8%) |
| Kanagawa | 5,402 (6.6%) |
| Koshin | 5,806 (7.1%) |
| Toyama | 4,587 (5.6%) |
| Aichi | 4,492 (5.5%) |
| Kyoto | 3,247 (4.0%) |
| Osaka | 6,268 (7.7%) |
| Hyogo | 4,185 (5.1%) |
| Tottori | 2,499 (3.1%) |
| Kochi | 5,487 (6.7%) |
| Fukuoka | 6,117 (7.5%) |
| South Kyushu/Okinawa | 4,455 (5.5%) |
Abbreviations: JPY Japanese yen, SD Standard deviation