Demographic characteristics of women in the study sample by BMI category (n=14 857)
| Variable | ≤18 kg/m2 (n=751) | 19–24 kg/m2 (n=7431) | 25–29 kg/m2 (n=3748) | 30–34 kg/m2 (n=1598) | 35–39 kg/m2 (n=737) | 40+ kg/m2 (n=592) |
|---|---|---|---|---|---|---|
| Maternal characteristics and outcomes | ||||||
| Mean age in years (SD) | 28.46 (5.848) | 30.53 (5.405) | 30.79 (5.627) | 30.63 (5.805) | 30.85 (5.383) | 30.99 (5.565) |
| Parity n (%) | ||||||
| 0 | 365 (50.3) | 3598 (48.4) | 1557 (41.5) | 664 (41.6) | 260 (35.3) | 210 (35.5) |
| 1 | 226 (31.2) | 2405 (32.4) | 1278 (34.1) | 512 (32.0) | 244 (33.1) | 190 (32.1) |
| 2 | 821 (1.3) | 979 (13.2) | 585 (15.6) | 244 (15.3) | 123 (16.7) | 102 (17.2) |
| 3 | 26 (3.6) | 294 (4.0) | 196 (5.2) | 110 (6.9) | 59 (8.0) | 49 (8.3) |
| ≥4 | 26 (3.6) | 155 (2.1) | 132 (3.5) | 68 (4.3) | 51 (6.9) | 41 (6.9) |
| Country of birth n (%) | ||||||
| Australian non-Aboriginal and Torres Strait Islander | 423 (58.3) | 4888 (65.8) | 2710 (72.3) | 1223 (76.5) | 608 (82.5) | 505 (85.3) |
| Australian Aboriginal and Torres Strait Islander | 26 (3.7) | 154 (2.1) | 85 (2.3) | 59 (3.8) | 26 (3.6) | 28 (4.9) |
| Asian | 190 (26.2) | 1241 (16.7) | 455 (12.1) | 113 (7.1) | 33 (4.5) | 8 (1.4) |
| Other | 84 (11.6) | 1128 (15.2) | 495 (13.2) | 201 (12.6) | 71 (9.6) | 51 (8.6) |
| Smoking status n (%) | ||||||
| Smoking ceased during pregnancy | 28 (3.9) | 277 (3.7) | 142 (3.8) | 71 (4.4) | 31 (4.2) | 34 (5.7) |
| Current smoker | 136 (18.8) | 660 (8.9) | 396 (10.6) | 323 (14.5) | 105 (14.2) | 94 (15.9) |
| Never smoker | 561 (77.4) | 6494 (87.4) | 3210 (85.6) | 1295 (81.0) | 601 (81.5) | 464 (78.4) |
| Maternal medical complications n (%) | ||||||
| Pre-existing diabetes (types I and II) | 5 (0.7) | 96 (1.3) | 90 (2.4) | 69 (4.3) | 47 (6.4) | 36 (6.1) |
| Essential hypertension | 2 (0.3) | 47 (0.6) | 46 (1.2) | 50 (3.1) | 38 (5.2) | 56 (9.5) |
| Birth status n (%) | ||||||
| Stillborn | 10 (1.38) | 86 (1.16) | 29 (0.77) | 23 (1.44) | 8 (1.09) | 6 (1.10) |
| Birth mode n (%) | ||||||
| Spontaneous vaginal | 489 (67.45) | 4744 (63.84) | 2239 (59.74) | 913 (57.13) | 385 (52.24) | 287 (48.48) |
| Caesarean | 119 (16.41) | 1636 (22.02) | 982 (26.2) | 520 (32.54) | 297 (40.3) | 238 (40.2) |
| Instrumental vaginal | 114 (15.72) | 1016 (13.67) | 510 (13.61) | 158 (9.89) | 54 (7.33) | 65 (10.98) |
| Apgar score n (%) | ||||||
| 5 min Apgar score ≤6 | 18 (2.49) | 268 (3.61) | 147 (3.93) | 97 (6.08) | 42 (5.7) | 36 (6.08) |
| Mean score (SD) | 8.85 (1.28) | 8.85 (1.31) | 8.80 (1.28) | 8.69 (1.51) | 8.68 (1.47) | 8.69 (1.35) |
| Infant birth weight n (%) | ||||||
| Macrosomia | 36 (4.97) | 751 (10.11) | 571 (15.23) | 271 (16.96) | 120 (16.28) | 116 (19.59) |
| Low birth weight <2500 g | 111 (15.31) | 653 (8.79) | 296 (7.9) | 172 (10.76) | 70 (9.5) | 49 (8.28) |
| Mean standardised z-score (SD) | −0.38 (1.00) | −0.04 (0.95) | 0.08 (0.99) | 0.05 (1.12) | 0.07 (1.08) | 0.16 (1.10) |
| SGA | 172 (22.90) | 970 (13.05) | 410 (10.94) | 178 (11.14) | 72 (9.78) | 54 (9.12) |
| LGA | 20 (2.66) | 500 (6.73) | 405 (10.81) | 218 (13.64) | 111 (15.10) | 107 (18.10) |
| Maternal blood loss n (%) | ||||||
| Haemorrhage ≥500 mL | 38 (5.2) | 406 (5.5) | 219 (5.8) | 107 (6.7) | 49 (6.6) | 32 (5.4) |
| Peristatus n (%) | ||||||
| Third-degree/fourth-degree tear* | 44 (7.26) | 300 (5.18) | 141 (5.10) | 37 (3.43) | 13 (2.95) | 10 (2.82) |
| Episiotomy* | 68 (11.22) | 569 (9.82) | 235 (8.50) | 66 (6.12) | 22 (5.00) | 18 (5.08) |
| Obstetric complications n (%) | ||||||
| GDM | 9 (1.24) | 118 (1.59) | 116 (3.09) | 75 (4.69) | 45 (6.11) | 52 (8.78) |
| Hypertensive disorders | 9 (1.24) | 8.2 (1.1) | 77 (2.05) | 58 (3.63) | 43 (5.83) | 53 (8.95) |
| Labour complications n (%) | ||||||
| Shoulder dystocia | 2 (0.28) | 76 (1.02) | 42 (1.12) | 21 (1.31) | 13 (1.76) | 11 (1.86) |
| VBAC | 2 (0.28) | 34 (0.46) | 11 (0.29) | 8 (0.5) | 0 (0.0) | 2 (0.34) |
| Neonatal outcomes n (%) | ||||||
| Hypoglycaemia | 12 (1.66) | 107 (1.44) | 75 (2.0) | 39 (2.44) | 24 (3.26) | 22 (3.72) |
| Birth trauma | 11 (1.52) | 94 (1.26) | 54 (1.44) | 26 (1.63) | 9 (1.22) | 12 (2.03) |
| Respiratory distress | 14 (1.93) | 187 (2.52) | 78 (2.08) | 55 (3.44) | 37 (5.02) | 24 (4.05) |
*Women who had a caesarean for the second pregnancy excluded.
BMI, body mass index; GDM, gestational diabetes mellitus; LGA, large for gestational age; SGA, small for gestational age; VBAC, vaginal birth after caesarean.