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Table 2 Maternal and child characteristics of 1: 1 matched cases and controls for preterm birth (PTB) and early-term birth (ETB), 1982–1988

From: Acute associations between heatwaves and preterm and early-term birth in 50 US metropolitan areas: a matched case-control study

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)
  1. Note: Matching factors are maternal race/ethnicity, maternal education, and MSA of residence of the mother
  2. 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