Characteristics of the study population
| N* | Mean (SD) or % | |
|---|---|---|
| Child's characteristics | ||
| Gestational age (weeks) | 4488 | 39.8 (1.2) |
| Birth weight (kg) | 4388 | 3.3 (0.4) |
| Weight at 3 months (kg) | 3739 | 5.8 (0.8) |
| Weight at 6 months (kg) | 3086 | 7.6 (0.9) |
| Weight at 12 months (kg) | 3946 | 9.7 (1.1) |
| Weight at 18 months (kg) | 2883 | 11.2 (1.3) |
| Sex | ||
| Male | 2303 | 51.3% |
| Female | 2189 | 48.7% |
| Siblings | ||
| Yes | 1237 | 27.5 |
| No | 3253 | 72.5 |
| Missing values | 2 | – |
| Wheezing from 6 to 18 months | ||
| Yes | 776 | 17.3% |
| No | 3716 | 82.7% |
| Lower respiratory tract infections† | ||
| Yes | 913 | 20.3% |
| No | 3579 | 79.7% |
| Maternal characteristics | ||
| Age (years) | 4492 | 33.6 (4.2) |
| Pre-pregnancy BMI | ||
| <18.5 | 390 | 8.9% |
| 18.5–24.99 | 3164 | 72.1% |
| 25+ | 833 | 19.0% |
| Missing values | 105 | – |
| Nationality | ||
| Italian | 4290 | 95.5% |
| Other | 202 | 4.5% |
| Educational level‡ | ||
| Low | 194 | 4.4% |
| Medium | 1466 | 32.9% |
| High | 2793 | 62.7% |
| Missing values | 39 | – |
| Smoking during pregnancy | ||
| Yes | 347 | 7.8% |
| No | 4092 | 92.2% |
| Missing values | 53 | – |
| Smoking after delivery§ | ||
| Yes | 588 | 13.2% |
| No | 3869 | 86.8% |
| Missing values | 35 | – |
| Asthma diagnosis | ||
| Yes | 350 | 8.0% |
| No | 4015 | 92.0% |
| Missing values | 127 | – |
*Total numbers may vary across variables due to missing values.
†Lower respiratory tract infections (bronchitis, bronchiolitis and pneumonia) between 6 and 18 months of age.
‡Low—primary school or less; medium—secondary school; and high—university degree.
§Maternal active smoking in the first 18 months after delivery.
BMI, body mass index.