Table 1 Characteristics of the study population by interpregnancy interval (IPI)
| Â | Interpregnancy interval | Â | |||||||
|---|---|---|---|---|---|---|---|---|---|
Variables | Total (n = 792,094) |  ≤ 11 month (n = 12,176) | 12–17 month (n = 56,716) | 18–23 month (n = 97,840) | 24–35 month (n = 186,588) | 36–59 month (n = 208,005) |  ≥ 60 month (n = 230,769) | Statistics | P |
Age, year, mean ± SD | 30.42 ± 5.36 | 26.53 ± 5.70 | 28.14 ± 5.76 | 29.69 ± 5.52 | 30.36 ± 5.33 | 30.32 ± 5.29 | 31.64 ± 4.90 | F = 6357.89 |  < 0.001 |
Race, n (%) | |||||||||
 White | 578,146 (72.99) | 8166 (67.07) | 41,196 (72.64) | 76,140 (77.82) | 145,666 (78.07) | 152,414 (73.27) | 154,564 (66.98) | χ2 = 12,146.74 |  < 0.001 |
 Black | 116,083 (14.66) | 2823 (23.18) | 9904 (17.46) | 11,748 (12.01) | 19,647 (10.53) | 27,413 (13.18) | 44,548 (19.30) |  |  |
 Other | 24,995 (3.16) | 578 (4.75) | 2355 (4.15) | 3031 (3.10) | 5362 (2.87) | 6262 (3.01) | 7407 (3.21) |  |  |
 Asian | 72,870 (9.20) | 609 (5.00) | 3261 (5.75) | 6921 (7.07) | 15,913 (8.53) | 21,916 (10.54) | 24,250 (10.51) |  |  |
Education level, n (%) | |||||||||
 Less than high school | 15,163 (1.91) | 239 (1.96) | 860 (1.52) | 1263 (1.29) | 2249 (1.21) | 3624 (1.74) | 6928 (3.00) | χ2 = 15,553.44 |  < 0.001 |
 High school | 233,851 (29.52) | 5902 (48.47) | 20,446 (36.05) | 24,433 (24.97) | 42,785 (22.93) | 58,589 (28.17) | 81,696 (35.40) |  |  |
 More than high school | 534,388 (67.47) | 5931 (48.71) | 34,881 (61.50) | 71,244 (72.82) | 139,611 (74.82) | 143,536 (69.01) | 139,185 (60.31) |  |  |
 Missing | 8692 (1.10) | 104 (0.85) | 529 (0.93) | 900 (0.92) | 1943 (1.04) | 2256 (1.08) | 2960 (1.28) |  |  |
Marital status, n (%) | |||||||||
 Married | 460,078 (58.08) | 5215 (42.83) | 30,791 (54.29) | 63,787 (65.20) | 125,514 (67.27) | 125,909 (60.53) | 108,862 (47.17) | χ2 = 26,705.16 |  < 0.001 |
 Unmarried | 224,320 (28.32) | 5787 (47.53) | 19,419 (34.24) | 22,027 (22.51) | 36,328 (19.47) | 53,066 (25.51) | 87,693 (38.00) |  |  |
 Missing | 107,696 (13.60) | 1174 (9.64) | 6506 (11.47) | 12,026 (12.29) | 24,746 (13.26) | 29,030 (13.96) | 34,214 (14.83) |  |  |
Weight gain, pounds, M (Q1, Q3) | 28.00 (19.00,37.00) | 26.00 (17.00,35.00) | 27.00 (18.00,35.00) | 29.00 (20.00,37.00) | 29.00 (20.00,37.00) | 28.00 (19.00,37.00) | 28.00 (18.00,38.00) | χ2 = 1245.86 |  < 0.001 |
Smoking before pregnancy, n (%) | |||||||||
 No | 733,610 (92.62) | 10,927 (89.74) | 52,364 (92.33) | 92,779 (94.83) | 177,324 (95.04) | 193,877 (93.21) | 206,339 (89.41) | χ2 = 6017.56 |  < 0.001 |
 Yes | 58,484 (7.38) | 1249 (10.26) | 4352 (7.67) | 5061 (5.17) | 9264 (4.96) | 14,128 (6.79) | 24,430 (10.59) |  |  |
Smoking during pregnancy, n (%) | |||||||||
 No | 747,537 (94.37) | 11,120 (91.33) | 53,186 (93.78) | 93,946 (96.02) | 179,767 (96.34) | 197,581 (94.99) | 211,937 (91.84) | χ2 = 5055.38 |  < 0.001 |
 Yes | 44,557 (5.63) | 1056 (8.67) | 3530 (6.22) | 3894 (3.98) | 6821 (3.66) | 10,424 (5.01) | 18,832 (8.16) |  |  |
Prenatal care, n (%) | |||||||||
 No | 6562 (0.83) | 230 (1.89) | 695 (1.23) | 805 (0.82) | 1240 (0.66) | 1485 (0.71) | 2107 (0.91) | χ2 = 389.80 |  < 0.001 |
 Yes | 785,532 (99.17) | 11,946 (98.11) | 56,021 (98.77) | 97,035 (99.18) | 185,348 (99.34) | 206,520 (99.29) | 228,662 (99.09) |  |  |
Pre-pregnancy BMI, kg/m2, mean ± SD | 28.66 ± 7.37 | 30.20 ± 7.88 | 29.06 ± 7.48 | 27.97 ± 7.11 | 27.80 ± 7.03 | 28.59 ± 7.32 | 29.54 ± 7.60 | F = 1487.17 |  < 0.001 |
Pre-pregnancy diabetes, n (%) | |||||||||
 No | 779,451 (98.40) | 11,951 (98.15) | 55,896 (98.55) | 96,627 (98.76) | 184,228 (98.74) | 204,914 (98.51) | 225,835 (97.86) | χ2 = 670.14 |  < 0.001 |
 Yes | 12,643 (1.60) | 225 (1.85) | 820 (1.45) | 1213 (1.24) | 2360 (1.26) | 3091 (1.49) | 4934 (2.14) |  |  |
Gestational diabetes, n (%) | |||||||||
 No | 725,733 (91.62) | 11,243 (92.34) | 52,746 (93.00) | 90,994 (93.00) | 173,293 (92.87) | 190,448 (91.56) | 207,009 (89.70) | χ2 = 1879.98 |  < 0.001 |
 Yes | 66,361 (8.38) | 933 (7.66) | 3970 (7.00) | 6846 (7.00) | 13,295 (7.13) | 17,557 (8.44) | 23,760 (10.30) |  |  |
Pre-pregnancy hypertension, n (%) | |||||||||
 No | 769,955 (97.21) | 11,751 (96.51) | 55,199 (97.33) | 95,701 (97.81) | 182,528 (97.82) | 202,763 (97.48) | 222,013 (96.21) | χ2 = 1327.37 |  < 0.001 |
 Yes | 22,139 (2.79) | 425 (3.49) | 1517 (2.67) | 2139 (2.19) | 4060 (2.18) | 5242 (2.52) | 8756 (3.79) |  |  |
Gestation hypertension, n (%) | |||||||||
 No | 741,155 (93.57) | 11,234 (92.26) | 53,157 (93.72) | 92,370 (94.41) | 176,039 (94.35) | 194,814 (93.66) | 213,541 (92.53) | χ2 = 752.12 |  < 0.001 |
 Yes | 50,939 (6.43) | 942 (7.74) | 3559 (6.28) | 5470 (5.59) | 10,549 (5.65) | 13,191 (6.34) | 17,228 (7.47) |  |  |
Eclampsia, n (%) | |||||||||
 No | 790,160 (99.76) | 12,139 (99.70) | 56,578 (99.76) | 97,636 (99.79) | 186,257 (99.82) | 207,509 (99.76) | 230,041 (99.68) | χ2 = 89.49 |  < 0.001 |
 Yes | 1934 (0.24) | 37 (0.30) | 138 (0.24) | 204 (0.21) | 331 (0.18) | 496 (0.24) | 728 (0.32) |  |  |
Assisted reproductive treatment, n (%) | |||||||||
 No | 782,824 (98.83) | 12,145 (99.75) | 56,377 (99.40) | 96,504 (98.63) | 183,671 (98.44) | 205,346 (98.72) | 228,781 (99.14) | χ2 = 741.76 |  < 0.001 |
 Yes | 9270 (1.17) | 31 (0.25) | 339 (0.60) | 1336 (1.37) | 2917 (1.56) | 2659 (1.28) | 1988 (0.86) |  |  |
Gestational age, week, mean ± SD | 38.52 ± 1.96 | 38.02 ± 2.55 | 38.53 ± 2.08 | 38.62 ± 1.88 | 38.65 ± 1.80 | 38.55 ± 1.89 | 38.36 ± 2.08 | F = 670.30 |  < 0.001 |
Clinical chorioamnionitis or maternal fever during labor, n (%) | |||||||||
 No | 786,200 (99.26) | 12,125 (99.58) | 56,379 (99.41) | 97,105 (99.25) | 185,227 (99.27) | 206,399 (99.23) | 228,965 (99.22) | χ2 = 41.94 |  < 0.001 |
 Yes | 5894 (0.74) | 51 (0.42) | 337 (0.59) | 735 (0.75) | 1361 (0.73) | 1606 (0.77) | 1804 (0.78) |  |  |
Previous preterm birth, n (%) | |||||||||
 No | 754,627 (95.27) | 11,241 (92.32) | 53,573 (94.46) | 93,460 (95.52) | 178,306 (95.56) | 198,460 (95.41) | 219,587 (95.15) | χ2 = 383.01 |  < 0.001 |
 Yes | 37,467 (4.73) | 935 (7.68) | 3143 (5.54) | 4380 (4.48) | 8282 (4.44) | 9545 (4.59) | 11,182 (4.85) |  |  |
Method of delivery, n (%) | |||||||||
Vaginal delivery | 87,850 (11.09) | 1010 (8.30) | 5515 (9.72) | 12,788 (13.07) | 24,303 (13.02) | 23,019 (11.07) | 21,215 (9.19) | χ2 = 2143.50 |  < 0.001 |
Cesarean delivery | 704,244 (88.91) | 11,166 (91.70) | 51,201 (90.28) | 85,052 (86.93) | 162,285 (86.98) | 184,986 (88.93) | 209,554 (90.81) | Â | Â |
Maternal adverse events, n (%) | |||||||||
 No | 786,848 (99.34) | 12,071 (99.14) | 56,301 (99.27) | 97,182 (99.33) | 185,404 (99.37) | 206,686 (99.37) | 229,204 (99.32) | χ2 = 17.29 | 0.004 |
 Yes | 5246 (0.66) | 105 (0.86) | 415 (0.73) | 658 (0.67) | 1184 (0.63) | 1319 (0.63) | 1565 (0.68) |  |  |
Maternal transfusion, n (%) | |||||||||
 No | 788,712 (99.57) | 12,112 (99.47) | 56,459 (99.55) | 97,387 (99.54) | 185,801 (99.58) | 207,156 (99.59) | 229,797 (99.58) | χ2 = 8.72 | 0.121 |
 Yes | 3382 (0.43) | 64 (0.53) | 257 (0.45) | 453 (0.46) | 787 (0.42) | 849 (0.41) | 972 (0.42) |  |  |
Ruptured uterus, n (%) | |||||||||
 No | 791,312 (99.90) | 12,156 (99.84) | 56,630 (99.85) | 97,719 (99.88) | 186,391 (99.89) | 207,814 (99.91) | 230,602 (99.93) | χ2 = 45.72 |  < 0.001 |
 Yes | 782 (0.10) | 20 (0.16) | 86 (0.15) | 121 (0.12) | 197 (0.11) | 191 (0.09) | 167 (0.07) |  |  |
Unplanned hysterectomy, n (%) | |||||||||
 No | 791,503 (99.93) | 12,169 (99.94) | 56,677 (99.93) | 97,769 (99.93) | 186,457 (99.93) | 207,846 (99.92) | 230,585 (99.92) | χ2 = 2.18 | 0.823 |
 Yes | 591 (0.07) | 7 (0.06) | 39 (0.07) | 71 (0.07) | 131 (0.07) | 159 (0.08) | 184 (0.08) |  |  |
Admission to ICU, n (%) | |||||||||
 No | 790,665 (99.82) | 12,150 (99.79) | 56,619 (99.83) | 97,698 (99.85) | 186,309 (99.85) | 207,636 (99.82) | 230,253 (99.78) | χ2 = 41.67 |  < 0.001 |
 Yes | 1429 (0.18) | 26 (0.21) | 97 (0.17) | 142 (0.15) | 279 (0.15) | 369 (0.18) | 516 (0.22) |  |  |
Neonatal adverse events, n(%) | |||||||||
 No | 647,671 (81.77) | 8891 (73.02) | 45,712 (80.60) | 81,515 (83.31) | 156,834 (84.05) | 171,927 (82.66) | 182,792 (79.21) | χ2 = 2610.49 |  < 0.001 |
 Yes | 144,423 (18.23) | 3285 (26.98) | 11,004 (19.40) | 16,325 (16.69) | 29,754 (15.95) | 36,078 (17.34) | 47,977 (20.79) |  |  |
Low birth weight, n (%) | |||||||||
 No | 749,746 (94.65) | 11,026 (90.56) | 53,728 (94.73) | 93,789 (95.86) | 179,043 (95.96) | 197,677 (95.03) | 214,483 (92.94) | χ2 = 2706.29 |  < 0.001 |
 Yes | 42,348 (5.35) | 1150 (9.44) | 2988 (5.27) | 4051 (4.14) | 7545 (4.04) | 10,328 (4.97) | 16,286 (7.06) |  |  |
Premature birth, n (%) | |||||||||
 No | 714,221 (90.17) | 10,049 (82.53) | 50,613 (89.24) | 89,233 (91.20) | 171,435 (91.88) | 188,987 (90.86) | 203,904 (88.36) | χ2 = 2554.33 |  < 0.001 |
 Yes | 77,873 (9.83) | 2127 (17.47) | 6103 (10.76) | 8607 (8.80) | 15,153 (8.12) | 19,018 (9.14) | 26,865 (11.64) |  |  |
Apgar score at 5 min < 7, n (%) | |||||||||
 No | 781,028 (98.60) | 11,905 (97.77) | 55,861 (98.49) | 96,595 (98.73) | 184,295 (98.77) | 205,258 (98.68) | 227,114 (98.42) | χ2 = 182.29 |  < 0.001 |
 Yes | 11,066 (1.40) | 271 (2.23) | 855 (1.51) | 1245 (1.27) | 2293 (1.23) | 2747 (1.32) | 3655 (1.58) |  |  |
Abnormal conditions of the newborn, n (%) | |||||||||
 No | 707,257 (89.29) | 10,334 (84.87) | 50,395 (88.85) | 88,285 (90.23) | 168,667 (90.40) | 186,685 (89.75) | 202,891 (87.92) | χ2 = 1088.64 |  < 0.001 |
 Yes | 84,837 (10.71) | 1842 (15.13) | 6321 (11.15) | 9555 (9.77) | 17,921 (9.60) | 21,320 (10.25) | 27,878 (12.08) |  |  |