| Characteristic | Reference births N (%) | SGA* births N (%) | p Value | VSGA† births N (%) | p Value |
| χ2 test | χ2 test | ||||
| (SGA vs Reference) | (VSGA vs Reference) | ||||
| Sample size | 178 198 (90) | 16 340 (8) | 4683 (2) | ||
| Maternal age (years) | <0.01 | <0.01 | |||
| <20 | 14 223 (8) | 2007 (13) | 597 (13) | ||
| 20–24 | 34 420 (19) | 3769 (23) | 1070 (23) | ||
| 25–29 | 45 010 (25) | 3947 (24) | 1103 (24) | ||
| 30–34 | 51 615 (29) | 3960 (24) | 1096 (23) | ||
| ⩾35 | 32 930 (19) | 2657 (16) | 817 (17) | ||
| Prenatal care initiation | <0.01 | <0.01 | |||
| First trimester | 141 153 (79) | 12 208 (75) | 3387 (72) | ||
| Second trimester | 29 605 (17) | 3218 (20) | 992 (21) | ||
| Third trimester | 7440 (4) | 914 (5) | 304 (7) | ||
| Maternal smoking during pregnancy | 14 350 (8) | 2384 (15) | <0.01 | 893 (19) | <0.01 |
| Maternal alcohol use during pregnancy | 1867 (1) | 297 (2) | <0.01 | 121 (3) | <0.01 |
| Maternal drug use during pregnancy | 2419 (1) | 553 (3) | <0.01 | 305 (7) | <0.01 |
| Maternal marital status | <0.01 | <0.01 | |||
| Single | 63 534 (36) | 7348 (45) | 2361 (51) | ||
| Married | 112 997 (63) | 8817 (54) | 2265 (48) | ||
| Separated, divorced, widowed | 1667 (1) | 175 (1) | 57 (1) | ||
| Maternal education | |||||
| < High school | 30 160 (17) | 3589 (22) | 1140 (24) | ||
| High school Graduate | 57 743 (32) | 5548 (34) | <0.01 | 1741 (37) | <0.01 |
| Some college or more | 90 295 (51) | 7203 (44) | 1802 (39) | ||
| Maternal race | |||||
| White (non-Hispanic) | 84 747 (47) | 7478 (46) | 1930 (41) | ||
| African–American (non-Hispanic) | 38 978 (22) | 3952 (24) | <0.01 | 1363 (29) | <0.01 |
| Hispanic | 54 473 (31) | 4910 (30) | 1390 (30) | ||
| Pregnancy complications | |||||
| Pre-pregnancy hypertension | 1990 (1) | 256 (2) | <0.01 | 139 (3) | <0.01 |
| Gestational hypertension | 10 028 (6) | 1452 (9) | <0.01 | 788 (17) | <0.01 |
| Pre-eclampsia | 3742 (2) | 688 (4) | <0.01 | 491 (10) | <0.01 |
| Eclampsia | 142 (0) | 15 (0) | 0.60 | 21 (0) | <0.01 |
| Diabetes mellitus (Type I and II) | 1491 (1) | 109 (1) | 0.02 | 47 (1) | 0.22 |
| Gestational diabetes | 8416 (5) | 574 (4) | <0.01 | 198 (4) | 0.11 |
| Placenta praevia | 766 (0) | 92 (1) | 0.01 | 53 (1) | <0.01 |
| Fetal distress | 9740 (5) | 1336 (8) | <0.01 | 600 (13) | <0.01 |
| Placental abruption | 839 (0) | 177 (1) | <0.01 | 136 (3) | <0.01 |
| Premature rupture of membrane | 5757 (3) | 797 (5) | <0.01 | 369 (8) | <0.01 |
| Percentage of population(⩾25 years) in maternal residence census tract with <12 years of education | <0.01 | <0.01 | |||
| <15 | 68 143 (38) | 5847 (36) | 1564 (33) | ||
| 15 to <25 | 39 325 (22) | 3686 (23) | 1015 (22) | ||
| 25 to <40 | 41 354 (23) | 3935 (24) | 1236 (26) | ||
| ⩾40 | 29 376 (17) | 2872 (17) | 868 (19) | ||
| Percentage of population(⩾25 years) in maternal residence census tract with at least 4 years of college education | <0.01 | <0.01 | |||
| <15 | 62 346 (35) | 6021 (37) | 1836 (39) | ||
| 15 to <25 | 41 095 (23) | 3846 (24) | 1111 (24) | ||
| 25 to <40 | 35 765 (20) | 3123 (19) | 871 (19) | ||
| ⩾40 | 38 992 (22) | 3350 (20) | 865 (18) | ||
| Percentage of population in maternal residence census tract below federally defined poverty line | <0.01 | <0.01 | |||
| <5 | 67 854 (38) | 5762 (35) | 1519 (32) | ||
| 5 to <10 | 35 328 (20) | 3334 (21) | 929 (20) | ||
| 10 to <20 | 40 086 (22) | 3759 (23) | 1110 (24) | ||
| ⩾20 | 34 930 (20) | 3485 (21) | 1125 (24) |
New Jersey Air Pollution and Adverse Birth Outcomes Study, 1999–2003 (N = 199,221 term births).
*SGA, “small for gestational age” defined as a fetal growth ratio (ie, newborn’s birthweight divided by median birthweight of corresponding gestational age/gender/race-specific stratum) ⩾0.75 and <0.85.
†VSGA, “very small for gestational age” defined as fetal growth ratio <0.75.