Table I.
Baseline characteristics of the study sample (n = 91 721).
| Characteristics | Number (%) | Mean (SD) |
|---|---|---|
| Parity | ||
| Primiparous | 43 593 (47.5) | |
| Multiparous | 48 128 (52.5) | |
| Maternal age (years) | 29.8 (4.6) | |
| BMI (kg/m2)a | 25.1 (4.2) | |
| Age at menarche (years)b | 13.0 (1.4) | |
| Planned pregnancy | ||
| Yes | 73 627 (80.3) | |
| No | 17 070 (18.6) | |
| Missing | 1024 (1.1) | |
| Hormonal contraceptive use last year before pregnancy | ||
| None | 52 310 (57.0) | |
| Combined oral contraceptive pills | 29 732 (32.4) | |
| Progestin-only contraceptive pills | 4972 (5.4) | |
| Progestin injections | 414 (0.5) | |
| Progestin intrauterine devices | 4293 (4.7) | |
| Oral contraceptive use last 4 months before pregnancyc | ||
| No hormonal contraceptives last year | 52 310 (60.1) | |
| Combined oral contraceptive pills | 15 810 (18.2) | |
| Progestin-only contraceptive pills | 2576 (3.0) | |
| Cessation of oral contraceptives | 16 318 (18.8) | |
| Oral contraceptive use at the time of being pregnantc | ||
| No hormonal contraceptives last year | 52 310 (60.1) | |
| Combined oral contraceptive pills | 1372 (1.6) | |
| Progestin-only contraceptive pills | 378 (0.4) | |
| Cessation of oral contraceptives | 32 954 (37.9) | |
BMI, body mass index; SD, standard deviation.
an = 87 216.
bn = 90 535.
cn = 87 014.