Maternal and infant characteristics | Birth year | ||||
2004–2006 (n=213 270) | 2007–2010 (n=301 508) | 2011–2013 (n=218 040) | P value* | Rate ratio (95% CI)† | |
n (%) | n (%) | n (%) | |||
Maternal age (years) | |||||
15–19 | 17 844 (8.4) | 24 380 (8.1) | 13 313 (6.1) | <0.001 | 0.73 (0.71 to 0.75) |
20–29 | 111 509 (52.3) | 157 445 (52.2) | 109 527 (50.2) | <0.001 | 0.96 (0.96 to 0.97) |
30–39 | 77 941 (36.6) | 111 085 (36.8) | 88 445 (40.6) | <0.001 | 1.11 (1.10 to 1.12) |
≥40 | 5976 (2.8) | 8598 (2.9) | 6755 (3.1) | <0.001 | 1.11 (1.07 to 1.14) |
Race | |||||
Non-Hispanic white | 153 872 (72.4) | 213 846 (71.1) | 155 618 (71.6) | <0.001 | 0.99 (0.99 to 0.99) |
African-American | 7775 (3.7) | 12 201 (4.1) | 9699 (4.5) | <0.001 | 1.22 (1.19 to 1.26) |
Native American | 4445 (2.1) | 6123 (2.0) | 4320 (2.0) | 0.006 | 0.95 (0.91 to 0.99) |
Hispanic | 25 871 (12.2) | 37 531 (12.5) | 23 387 (10.8) | <0.001 | 0.88 (0.87 to 0.90) |
Other | 20 649 (9.7) | 31 131 (10.3) | 24 324 (11.2) | <0.001 | 1.15 (1.13 to 1.17) |
Maternal education (<high school) | 9800 (4.6) | 11 318 (3.7) | 6032 (2.8) | <0.001 | 0.60 (0.58 to 0.62) |
Smoking during pregnancy | 20 834 (9.9) | 29 206 (9.8) | 19 635 (9.0) | <0.001 | 0.91 (0.89 to 0.93) |
Not married | 67 109 (31.6) | 104 035 (34.6) | 75 055 (34.5) | <0.001 | 1.09 (1.08 to 1.10) |
Parity | |||||
Nullipara | 85 407 (40.0) | 124 799 (41.4) | 88 428 (40.6) | 0.010 | 1.01 (1.01 to 1.02) |
Para 1–3 | 110 757 (51.9) | 159 444 (52.9) | 117 743 (54.0) | <0.001 | 1.04 (1.03 to 1.05) |
Grand multipara (≥4 births) | 17 106 (8.0) | 17 265 (5.7) | 11 869 (5.4) | <0.001 | 0.68 (0.66 to 0.69) |
Body mass index (kg/m2) | |||||
Underweight (<18.5) | 6240 (2.9) | 8332 (2.8) | 6230 (2.9) | 0.045 | 0.98 (0.94 to 1.00) |
Normal (18.5 to 24.9) | 89 369 (41.9) | 131 590 (43.6) | 94 093 (43.1) | <0.001 | 1.03 (1.02 to 1.04) |
Overweight (25 to 29.9) | 46 074 (21.6) | 72 295 (24.0) | 53 849 (24.7) | <0.001 | 1.14 (1.13 to 1.16) |
Obese (≥30) | 39 752 (18.6) | 65 239 (21.6) | 51 806 (23.8) | <0.001 | 1.27 (1.26 to 1.29) |
Missing | 31 842 (14.9) | 24 052 (8.0) | 12 062 (5.5) | <0.001 | |
Assisted conception | 1470 (0.7) | 2702 (0.90) | 2249 (1.0) | <0.001 | 1.47 (1.38 to 1.57) |
Diabetes mellitus (types 1 and 2) | 1578 (0.7) | 2591 (0.9) | 2151 (1.0) | <0.001 | 1.33 (1.25 to 1.42) |
Gestational diabetes | 12 255 (5.75) | 19 160 (6.3) | 16 382 (7.5) | <0.001 | 1.31 (1.28 to 1.34) |
Chronic hypertension | 2489 (1.2) | 3783 (1.3) | 2815 (1.3) | <0.001 | 1.09 (1.03 to 1.15) |
Hypertension in pregnancy | 10 857 (5.1) | 15 304 (5.1) | 12 010 (5.5) | <0.001 | 1.08 (1.06 to 1.11) |
Infant sex (male) | 109 744 (51.5) | 154 777 (51.3) | 112 326 (51.5) | 0.846 | 1.00 (1.00 to 1.01) |
Preterm birth | |||||
<34 weeks | 3042 (1.4) | 4372 (1.4) | 3255 (1.5) | 0.104 | 1.05 (1.00 to 1.10) |
<37 weeks | 15 006 (7.0) | 20 067 (6.7) | 14 265 (6.5) | <0.001 | 0.93 (0.91 to 0.95) |
Birth weight (g) | |||||
<2500 | 9414 (4.4) | 13 448 (4.5) | 10 186 (4.7) | <0.001 | 1.06 (1.03 to 1.09) |
2500–2999 | 29 718 (14.0) | 43 033 (14.3) | 31 990 (14.7) | <0.001 | 1.05 (1.04 to 1.07) |
3000–3999 | 149 045 (70.1) | 211 901 (70.3) | 151 953 (69.7) | 0.002 | 1.00 (0.99 to 1.00) |
4000–4499 | 20 754 (9.8) | 28 051 (9.3) | 20 094 (9.2) | <0.001 | 0.95 (0.93 to 0.96) |
≥4500 | 3731 (1.75) | 4819 (1.6) | 3640 (1.7) | 0.005 | 0.95 (0.91 to 1.00) |
Caesarean delivery | |||||
Primary with labour | 19 965 (9.4) | 27 960 (9.3) | 18 323 (8.4) | <0.001 | 0.90 (0.88 to 0.92) |
Primary no labour | 16 310 (7.6) | 23 314 (7.7) | 15 930 (7.3) | <0.001 | 0.96 (0.94 to 0.98) |
Repeat with labour | 1227 (0.6) | 1939 (0.6) | 1961 (0.9) | <0.001 | 1.56 (1.46 to 1.68) |
Repeat no labour | 16 931 (7.9) | 28 281 (9.4) | 22 366 (10.3) | <0.001 | 1.29 (1.27 to 1.32) |
Vaginal delivery | |||||
Spontaneous | 144 931 (68.0) | 203 513 (67.5) | 149 270 (68.5) | <0.001 | 1.01 (1.00 to 1.01) |
Forceps | 1698 (0.8) | 2185 (0.7) | 1416 (0.6) | <0.001 | 0.82 (0.76 to 0.88) |
Vacuum | 12 208 (5.7) | 14 316 (4.7) | 8774 (4.0) | <0.001 | 0.70 (0.68 to 0.72) |
Previous caesarean delivery | |||||
One prior delivery | 16 198 (7.6) | 25 871 (8.6) | 21 049 (9.6) | <0.001 | 1.27 (1.25 to 1.30) |
Two or more | 4518 (2.1) | 8821 (2.9) | 8069 (3.7) | <0.001 | 1.75 (1.68 to 1.81) |
Labour induction | 50 584 (24.2) | 69 022 (23.1) | 48 530 (22.3) | <0.001 | 0.92 (0.91 to 0.93) |
PROM (>12 hours) | 8435 (4.1) | 16 232 (5.5) | 13 246 (6.1) | <0.001 | 1.50 (1.46 to 1.54) |
Precipitous labour (<3 hours) | 6323 (3.1) | 9595 (3.2) | 6486 (3.0) | 0.475 | 0.98 (0.95 to 1.01) |
Prolonged labour | 3206 (1.5) | 7759 (2.6) | 5691 (2.6) | <0.001 | 1.69 (1.62 to 1.77) |
Cephalopelvic disproportion | 6669 (3.1) | 7873 (2.6) | 4374 (2.0) | <0.001 | 0.64 (0.62 to 0.67) |
Obstructed labour | 16 256 (7.6) | 21 224 (7.0) | 13 524 (6.2) | <0.001 | 0.81 (0.80 to 0.83) |
Breech delivery excluded.
↵*Cochran-Armitage test for trend.
†Rate ratio comparing periods 2011–2013 versus 2004–2006.
PROM, premature rupture of membranes.