Baseline and birth characteristics of the overall cohort and the cohort matched on high-dimensional propensity scores (HDPS)*
| Characteristic | Overall cohort n = 187 605 | HDPS-matched cohort n = 129 674 | ||||
|---|---|---|---|---|---|---|
| Unexposed n = 117 051 | Exposed n = 70 554 | Standardized difference | Unexposed n = 64 837 | Exposed n = 64 837 | Standardized difference | |
| Sex, male | 59 894 (51.2) | 36 172 (51.3) | 0.002 | 33 358 (51.5) | 33 358 (51.5) | 0.000 |
| Urban region of residence | 65 305 (55.8) | 36 418 (51.6) | 0.084 | 33 417 (51.5) | 33 488 (51.7) | 0.002 |
| Socioeconomic status† | 0.174 | 0.066 | ||||
| High | 12 873 (11.0) | 5340 (7.6) | 5995 (9.3) | 5223 (8.1) | ||
| Middle | 44 027 (37.6) | 23 105 (32.8) | 23 084 (35.6) | 22 065 (34.0) | ||
| Low middle | 35 291 (30.2) | 23 319 (33.1) | 20 448 (31.5) | 21 425 (33.0) | ||
| Low | 24 860 (21.2) | 18 790 (26.6) | 15 310 (23.6) | 16 124 (24.9) | ||
| Receiving income assistance‡ | 18 503 (15.8) | 20 652 (29.3) | 0.326 | 15 681 (24.2) | 16 717 (25.8) | 0.037 |
| Maternal age at delivery, mean ± SD | 28.7 ± 5.8 | 27.6 ± 5.9 | 0.185 | 27.8 ± 5.8 | 27.7 ± 5.9 | 0.008 |
| Breastfeeding initiation§ | 95 919 (82.3) | 54 251 (77.2) | 0.127 | 51 865 (80.4) | 50 709 (78.5) | 0.045 |
| Preterm delivery¶ | 8139 (7.0) | 5578 (7.9) | 0.036 | 4694 (7.3) | 4962 (7.7) | 0.016 |
| Cesarean delivery | 23 213 (19.8) | 14 654 (20.8) | 0.023 | 12 837 (19.8) | 13 571 (20.9) | 0.028 |
| First-born child | 45 645 (39.0) | 24 946 (35.4) | 0.075 | 24 272 (37.4) | 23 731 (36.6) | 0.017 |
| Small for gestational age** | 9046 (7.7) | 5273 (7.5) | 0.010 | 4943 (7.6) | 4795 (7.4) | 0.009 |
| Birth complications†† | 11 743 (10.0) | 7150 (10.1) | 0.003 | 6356 (9.8) | 6623 (10.2) | 0.014 |
| Prenatal smoking‡‡ | 12 296 (15.4) | 11 801 (25.3) | 0.247 | 8518 (19.7) | 10 131 (23.4) | 0.089 |
| Prenatal alcohol or drug use§§ | 8948 (11.4) | 7117 (15.5) | 0.121 | 5719 (13.4) | 6270 (14.7) | 0.037 |
| Maternal medical conditions | ||||||
| Mood and anxiety disorders | 17 082 (14.6) | 16 502 (23.4) | 0.226 | 11 930 (18.4) | 13 567 (20.9) | 0.064 |
| Asthma | 5557 (4.8) | 8399 (11.9) | 0.261 | 4083 (6.3) | 6854 (10.8) | 0.154 |
| Developmental disabilities | 1994 (1.7) | 2029 (2.9) | 0.078 | 1382 (2.1) | 1689 (2.6) | 0.031 |
Note: SD = standard deviation.
↵* Data are presented as number (percentage) of children, unless stated otherwise. The percentages were calculated on the basis of non-missing data.
↵† Based on the Socio-Economic Factor Index, a neighbourhood-level measure that is based on the Canadian census, categorized with cut points within 1 SD from the mean into high, middle, low middle and low socioeconomic status.
↵‡ Defined as receiving income assistance for at least 2 months within 12 months before to 18 months after the index date.
↵§ Data missing for 789 children (0.4%) in the overall cohort and 554 (0.4%) children in the matched cohort.
↵¶ Defined as gestational age less than 37 weeks. Gestational age was based on the first date of a woman’s last menstrual period. Data missing for 193 children (0.1%) in the overall cohort and 133 (0.1%) children in the matched cohort.
↵** Defined as birth weight below the 10th percentile for gestational age and sex. Data missing for 264 children (0.1%) in the overall cohort and 186 children (0.1%) in the matched cohort.
↵†† Includes vacuum, forceps or breech procedures (clinical modification of International Classification of Diseases, 9th Revision [ICD-9-CM] procedure codes 72, 73.3, 763.2, 652.2, 669.6; Canadian Classification of Health Interventions [CCI] codes 5MD53, 5MD54, 5MD55 and 5MD56), shoulder dystocia (ICD-9-CM code 660.4), placental abruption (ICD-9-CM code 641.2), cord prolapse (ICD-9-CM code 663), uterine rupture (ICD-9-CM code 665.1) or other perinatal complications (ICD-9-CM codes 760–779).
↵‡‡ Data missing for 61 390 children (32.7%) in the overall cohort and 43 092 children (33.2%) in the matched cohort.
↵§§ Data missing for 63 238 children (33.7%) in the overall cohort and 44 351 children (34.2%) in the matched cohort.