Table 2.
Maternal and child characteristics of 1: 1 matched cases and controls for preterm birth (PTB) and early-term birth (ETB), 1982–1988
| Outcome | Preterm birth assessment | Early-term birth assessment | ||
|---|---|---|---|---|
| PTB Case (N = 615,329) | Control (N = 615,329) | ETB Case (N = 1,005,576) | Control (N = 1,005,576) | |
| Maternal race/ethnicity, n (%) | ||||
| Hispanic | 88,103 (14.3) | 88,103 (14.3) | 137,747 (13.7) | 137,747 (13.7) |
| Non-Hispanic white | 299,096 (48.6) | 299,096 (48.6) | 593,439 (59.0) | 593,439 (59.0) |
| Non-Hispanic black | 202,327 (32.9) | 202,327 (32.9) | 225,067 (22.4) | 225,067 (22.4) |
| Other | 23,868 (3.9) | 23,868 (3.9) | 46,115 (4.6) | 46,115 (4.6) |
| Missing | 1935 (0.3) | 1935 (0.3) | 3208 (0.3) | 3208 (0.3) |
| Maternal years of education, n (%) | ||||
| ≤ 12 | 130,493 (21.2) | 130,493 (21.2) | 150,163 (14.9) | 150,163 (14.9) |
| 12 | 178,579 (29.0) | 178,579 (29.0) | 289,889 (28.8) | 289,889 (28.8) |
| ≥ 13 | 131,850 (21.4) | 131,850 (21.4) | 280,117 (27.9) | 280,117 (27.9) |
| Missing | 174,407 (28.4) | 174,407 (28.4) | 285,407 (28.4) | 285,407 (28.4) |
| Region of residence, n (%) | ||||
| Northeast | 141,127 (23.0) | 141,127 (23.0) | 245,385 (24.4) | 245,385 (24.4) |
| Midwest | 135,587 (22.0) | 135,587 (22.0) | 222,753 (22.2) | 222,753 (22.2) |
| South | 207,442 (33.7) | 207,442 (33.7) | 310,046 (30.8) | 310,046 (30.8) |
| West | 131,173 (21.3) | 131,173 (21.3) | 227,392 (22.6) | 227,392 (22.6) |
| Maternal age (year), n (%) | ||||
| ≤ 20 | 109,251 (17.8) | 94,273 (15.3) | 117,589 (11.7) | 120,210 (12.0) |
| 21–34 | 458,365 (74.5) | 479,083 (77.9) | 800,633 (79.6) | 813,498 (80.9) |
| ≥ 35 | 47,713 (7.7) | 41,973 (6.8) | 87,354 (8.7) | 71,868 (7.1) |
| Child sex, n (%) | ||||
| Male | 328,851 (53.4) | 313,212 (50.9) | 533,361 (53.0) | 507,237 (50.4) |
| Marital status, n (%) | ||||
| Married | 375,376 (61.0) | 411,275 (66.8) | 740,573 (73.7) | 745,228 (74.1) |
| Unmarried | 239,779 (39.0) | 203,842 (33.1) | 264,718 (26.3) | 260,062 (25.9) |
| Missing | 174 (0.03) | 212 (0.03) | 285 (0.03) | 286 (0.03) |
| Month of last normal menses, n (%) | ||||
| January | 80,971 (13.2) | 88,654 (14.4) | 84,520 (8.4) | 91,597 (9.1) |
| February | 88,635 (14.4) | 82,874 (13.5) | 0a | 0a |
| March | 40,652 (6.6) | 44,981 (7.3) | 0a | 0a |
| August | 39,018 (6.3) | 36,480 (5.9) | 153,484 (12.3) | 153,115 (15.2) |
| September | 78,975 (12.8) | 76,831 (12.5) | 163,803 (16.3) | 162,379 (16.2) |
| October | 97,016 (15.8) | 95,043 (15.4) | 198,497 (19.7) | 199,641 (19.9) |
| November | 96,203 (15.6) | 92,861 (15.1) | 198,761 (19.8) | 193,311 (19.2) |
| December | 93,859 (15.3) | 97,677 (15.9) | 206,511 (20.5) | 205,533 (20.4) |
Note: Matching factors are maternal race/ethnicity, maternal education, and MSA of residence of the mother
a Women whose LMP date between April–July (and February–July for early-term) were not eligible for the study because the at-risk window for preterm birth was not in the warm season