Table 1 Characteristics of the study population by BMI categories
From: Pre-pregnancy BMI and weight gain: where is the tipping point for preterm birth?
Characteristics | Total (N = 45,824) | Underweight BMI <18.50 N = 4,348 (9.5%) | Normal BMI 18.50-24.99 N = 31,555 (68.9%) | Overweight BMI 25.00-29.99 N = 6,961 (15.2%) | Obese BMI ≥ 30.00 N = 2,960 (6.5%) |  |
|---|---|---|---|---|---|---|
N (%) | N (%) | N (%) | N (%) | N (%) | p-value | |
Mean Age (SD) | 24.1 (6.0) | 22.4 (5.2) | 23.4 (5.7) | 26.3 (6.5) | 28.1 (6.6) | <0.0001 |
Marital | Â | Â | Â | Â | Â | <0.0001 |
 Single | 7075 (15.4) | 794 (18.3) | 5089 (16.1) | 874 (12.6) | 318 (10.7) |  |
 Married | 34913 (76.2) | 3220 (74.1) | 24004 (76.1) | 5416 (77.8) | 2273 (76.8) |  |
 Other | 3834 (8.4) | 334 (7.7) | 2460 (7.8) | 671 (9.6) | 369 (12.5) |  |
 Missing | 2 (0.0) | 0 (0.0) | 2 (0.0) | 0 (0.0) | 0 (0.0) |  |
Education | Â | Â | Â | Â | Â | <0.0001 |
 Less than High School | 26066 (56.9) | 2528 (58.1) | 17193 (54.5) | 4329 (62.2) | 2016 (68.1) |  |
 High School | 13753(30.0) | 1311 (30.2) | 9662 (30.6) | 2019 (29.0) | 761 (25.7) |  |
 More than High School | 5204 (11.4) | 432 (9.9) | 4146 (13.1) | 491 (7.1) | 135 (4.6) |  |
 Missing | 801(1.8) | 77 (1.8) | 554 (1.8) | 122 (1.8) | 48 (1.6) |  |
Race | Â | Â | Â | Â | Â | <0.0001 |
 White | 20341 (44.4) | 2009 (46.2) | 14768 (46.8) | 2568 (36.9) | 996 (33.7) |  |
 Black | 22231 (48.5) | 1989 (45.8) | 14538 (46.1) | 3896 (56.0) | 1808 (61.1) |  |
 Other | 3071 (6.7) | 296 (6.8) | 2126 (6.7) | 495 (7.1) | 154 (5.2) |  |
 Missing | 181 (0.4) | 54 (1.2) | 123 (0.4) | 2 (0.0) | 2 (0.1) |  |
SES Index | Â | Â | Â | Â | Â | <0.0001 |
 0.0-3.9 | 17850 (39.0) | 1803 (41.5) | 11619 (36.8) | 3016 (43.3) | 1412 (47.7) |  |
 4.0-5.9 | 13967 (30.5) | 1295 (29.8) | 9371 (29.7) | 2033 (32.8) | 998 (33.7) |  |
 6.0-9.5 | 12830 (28.0) | 1138 (26.2) | 9764 (30.9) | 1456 (20.9) | 472 (16.0) |  |
 Missing | 1177 (2.6) | 112 (2.6) | 801 (2.5) | 186 (2.7) | 78 (2.6) |  |
Tobacco use | 21154 (46.2) | 2257 (51.9) | 14819(47.0) | 2907 (41.8) | 1171 (39.6) | <0.0001 |
Missing | 280 (0.6) | 23 (0.5) | 192 (0.6) | 39 (0.6) | 26 (0.9) | Â |
Substance use | 50 (0.1) | 1 (0.0) | 43 (0.1) | 5 (0.1) | 1 (0.0) | 0.2416 |
Missing | 283 (0.6) | 24 (0.6) | 195 (0.6) | 45 (0.7) | 19 (0.6) | Â |
Parity | Â | Â | Â | Â | Â | <0.0001 |
 Primiparous | 13028 (28.4) | 1567 (36.0) | 10010 (31.7) | 1121 (16.1) | 330 (11.2) |  |
 Multiparous | 32635 (71.2) | 2758 (63.4) | 21439 (67.9) | 5816 (83.6) | 2622 (88.6) |  |
 Missing | 161 (0.4) | 23 (0.5) | 106 (0.3) | 24 (0.3) | 8 (0.3) |  |
Previous Premature infant | Â | Â | Â | Â | Â | <0.0001 |
 Previous Preterm birth | 7602 (16.6) | 874 (20.1) | 4915(15.6) | 1218 (17.5) | 595 (20.1) |  |
 Previous Full-term birth | 25054 (54.7) | 1885 (43.4) | 16537 (52.4) | 4604 (66.1) | 2028 (68.5) |  |
 No Previous births | 13067 (28.5) | 1577 (36.3) | 10037 (31.8) | 1122 (16.1) | 331 (11.2) |  |
 Missing | 101 (0.2) | 12 (0.3) | 66 (0.2) | 17 (0.2) | 6 (0.2) |  |
Number of Prenatal Visits | Â | Â | Â | Â | Â | <0.0001 |
 1-5 | 10158 (22.2) | 1016 (23.4) | 6903 (21.9) | 1514 (21.8) | 725 (24.5) |  |
 6-9 | 16326 (35.6) | 1611 (37.1) | 11171 (35.4) | 2495 (35.8) | 1049 (35.4) |  |
 10-14 | 15854 (34.6) | 1445 (33.2) | 11036 (35.0) | 2428 (34.8) | 945 (31.9) |  |
 15 and above | 3446 (7.5) | 270 (6.2) | 2421 (7.7) | 515 (7.4) | 240 (8.1) |  |
 Missing | 40 (0.1) | 6 (0.1) | 24 (0.1) | 9 (0.1) | 1 (0.0) |  |
Diabetes | 223 (0.5) | 15 (0.3) | 97 (0.3) | 54 (0.8) | 57 (1.9) | <0.0001 |
Missing | 256 (0.6) | 28 (0.6) | 173 (0.6) | 37 (0.5) | 18 (0.6) | Â |
Hypertension | 2172 (4.7) | 101 (2.3) | 1062 (3.4) | 503 (7.2) | 506 (17.1) | <0.0001 |
Missing | 94 (0.2) | 5 (0.1) | 67 (0.2) | 14 (0.2) | 8 (0.3) | Â |
Anemia | 9153 (20.0) | 980 (22.5) | 6366 (20.2) | 1323 (19.0) | 484 (16.4) | <0.0001 |
Missing | 222 (0.5) | 21 (0.5) | 152 (0.5) | 34 (0.5) | 15 (0.5) | Â |
Preelampsia | 6380 (14.0) | 425 (9.8) | 3971 (12.6) | 1216 (17.5) | 768 (26.0) | <0.0001 |
Missing | 65 (0.1) | 4 (0.1) | 44 (0.1) | 11 (0.2) | 6 (0.2) | Â |
Weight gain (kg) | Â | Â | Â | Â | Â | <0.0001 |
 < 7.0 | 12863 (28.1) | 767 (17.6) | 7851 (24.9) | 2651 (38.1) | 1594 (53.9) |  |
 7.0-9.4 | 9254 (20.2) | 905 (20.8) | 6721 (21.3) | 1234 (17.7) | 384 (13.3) |  |
 9.5-12.7 | 12270 (26.8) | 1387 (31.9) | 9046 (28.7) | 1447 (20.8) | 390 (13.2) |  |
 > 12.7 | 11437 (25.0) | 1289 (29.7) | 7937 (25.2) | 1629 (23.4) | 582 (19.7) |  |
Delivery method | Â | Â | Â | Â | Â | <0.0001 |
 Vaginal | 43422 (94.8) | 4171 (95.9 | 30073 (95.3) | 6506 (93.5) | 2672 (90.3) |  |
 Cesarean | 2401 (5.2) | 177 (4.1) | 1481 (4.7) | 455 (6.5) | 288 (9.3) |  |
 Missing | 1 (0.0) | 0 (0.0) | 1 (0.0) | 0 (0.0) | 0 (0.0) |  |
Birth weight | Â | Â | Â | Â | Â | <0.0001 |
 Low (<2500 grams) | 4645 (10.1) | 713 (16.4) | 3259 (10.3) | 478 (6.9) | 195 (6.6) |  |
 Normal (≥ 2500 grams) | 41151 (89.8) | 3633 (83.6) | 28282 (89.6) | 6477 (93.1) | 2759 (93.2) |  |
 Missing | 28 (0.1) | 2 (0.1) | 14 (0.0) | 6 (0.1) | 6 (0.2) |  |
Gestational Age | Â | Â | Â | Â | Â | <0.0001 |
 Spontaneous PTB with PROM | 3526 (7.7) | 384 (8.8) | 2421 (7.7) | 508 (7.3) | 213 (7.2) |  |
 Spontaneous PTB no PROM | 2278 (5.0) | 279 (6.4) | 1595 (5.1) | 283 (4.1) | 121 (4.1) |  |
 Indicated PTB | 557 (1.2) | 48 (1.1) | 381 (1.2) | 80 (1.2) | 48 (1.6) |  |
 Full Term Birth | 39463 (86.1) | 3637 (83.7) | 27158 (86.1) | 6090 (87.5) | 2578 (87.1) |  |