Table 1.
Characteristics of mothers of twin and singleton infants, National Birth Defects Prevention Study, 1997–2007
| Maternal characteristics | Controls | Cases | ||||
|---|---|---|---|---|---|---|
|
|
|
|||||
| Twins (n=218) n (%) |
Singletons (n=7654) n (%) |
p Value* | Twins (n=1182) n (%) |
Singletons (n=19 627) n (%) |
p Value* | |
| Any fertility treatment used | <0.01 | <0.01 | ||||
| No | 147 (67.4) | 7408 (96.8) | 837 (70.8) | 18 798 (95.8) | ||
| Yes | 71 (32.6) | 246 (3.2) | 345 (29.2) | 829 (4.2) | ||
| Assisted reproductive technology | 39 (17.9) | 39 (0.5) | <0.01 | 178 (15.1) | 166 (0.9) | <0.01 |
| Clomiphene citrate | 19 (8.7) | 130 (1.7) | <0.01 | 118 (10.0) | 433 (2.2) | <0.01 |
| Age at delivery (years) | <0.01 | <0.01 | ||||
| <25 | 38 (17.4) | 2554 (33.4) | 276 (23.4) | 6778 (34.5) | ||
| 25–29 | 49 (22.5) | 2105 (27.5) | 288 (24.4) | 5209 (26.5) | ||
| 30–34 | 77 (35.3) | 1922 (25.1) | 355 (30.0) | 4763 (24.3) | ||
| >34 | 54 (24.8) | 1073 (14.0) | 263 (22.3) | 2877 (14.7) | ||
| Race/ethnicity | <0.01 | <0.01 | ||||
| Non-Hispanic white | 153 (70.2) | 4683 (61.2) | 786 (66.5) | 11 944 (60.9) | ||
| Non-Hispanic black | 31 (14.2) | 882 (11.5) | 141 (11.9) | 2025 (10.3) | ||
| Hispanic | 23 (10.6) | 1583 (20.7) | 168 (14.2) | 4320 (22.0) | ||
| Other | 11 (5.0) | 506 (6.6) | 87 (7.3) | 1338 (6.8) | ||
| Smoking† | 0.88 | <0.01 | ||||
| Yes | 41 (18.8) | 1471 (19.2) | 194 (16.4) | 4300 (21.9) | ||
| No | 177 (81.2) | 6183 (80.8) | 988 (83.6) | 15 327 (78.1) | ||
| Prepregnancy body mass index (kg/m2) | 0.80 | 0.62 | ||||
| <30 | 180 (82.6) | 6370 (83.2) | 945 (80.0) | 15 808 (80.5) | ||
| ≥30 | 38 (17.4) | 1284 (16.8) | 237 (20.1) | 3819 (19.5) | ||
| Parity | 0.66 | 0.17 | ||||
| No previous live births | 91 (41.7) | 3082 (40.3) | 533 (45.1) | 8450 (43.1) | ||
| One or more previous live births | 127 (58.3) | 4572 (59.7) | 649 (54.9) | 11 177 (57.0) | ||
| Education | <0.01 | <0.01 | ||||
| High school graduate or less | 57 (26.2) | 3047 (39.8) | 363 (30.7) | 8485 (43.2) | ||
| More than high school graduate | 161 (73.9) | 4607 (60.2) | 819 (69.3) | 11 142 (56.8) | ||
| Folic acid-containing multivitamin use‡ | <0.01 | <0.01 | ||||
| Yes | 136 (62.4) | 3979 (52.0) | 744 (62.9) | 9896 (50.4) | ||
| No | 82 (37.6) | 3675 (48.0) | 438 (37.1) | 9731 (49.6) | ||
| Study site | <0.01 | <0.01 | ||||
| Arkansas | 19 (8.7) | 988 (12.9) | 132 (11.2) | 2681 (13.7) | ||
| California | 6 (2.8) | 910 (11.9) | 114 (9.6) | 2548 (13.0) | ||
| Georgia | 23 (10.6) | 801 (10.5) | 141 (11.9) | 2197 (11.2) | ||
| Iowa | 35 (16.1) | 867 (11.3) | 93 (7.9) | 1911 (9.7) | ||
| Massachusetts | 51 (23.4) | 959 (12.5) | 240 (20.3) | 2581 (13.2) | ||
| New Jersey | 26 (11.9) | 479 (6.3) | 96 (8.1) | 1366 (7.0) | ||
| New York | 11 (5.1) | 682 (5.1) | 103 (8.7) | 1334 (6.8) | ||
| North Carolina | 14 (6.4) | 518 (6.8) | 58 (4.9) | 1057 (5.4) | ||
| Texas | 19 (8.7) | 868 (11.3) | 108 (9.1) | 2315 (11.8) | ||
| Utah | 14 (6.4) | 582 (7.6) | 97 (8.2) | 1057 (8.3) | ||
*
Mantel-Haenszel χ2 statistics were used to compare group differences for each characteristic of interest.
†
Any reported use 1 month prior to pregnancy through 3 months after conception.
‡
Any reported use 1 month prior to pregnancy through the first month of pregnancy.