TABLE 1.
Characteristics of the Study Population According to Plurality, 1989–1991 and 1999–2001
| Characteristic | Singletons
|
Twins
|
Triplets
|
Quadruplets and Quintupletsa
|
All Pluralities
|
|||||
|---|---|---|---|---|---|---|---|---|---|---|
| 1989–1991 | 1999–2001 | 1989–1991 | 1999–2001 | 1989–1991 | 1999–2001 | 1989–1991 | 1999–2001 | 1989–1991 | 1999–2001 | |
| No. of mothersb | 11 044 953 | 10 820 505 | 132 185 | 169 934 | 2648 | 6395 | 152 | 375 | 11 179 938 | 10 997 209 |
| Maternal age, mean (SD), y | 26.4 (5.7) | 27.1 (6.2) | 27.7 (5.6) | 29.2 (6.2) | 29.8 (4.8) | 31.9 (5.2) | 29.9 (4.3) | 31.3 (4.4) | 26.4 (5.7) | 27.2 (6.2) |
| Maternal age groups, % | ||||||||||
| <20 | 12.8 | 11.9 | 7.7 | 6.0 | 2.6 | 1.2 | 1.4 | 0.5 | 12.7 | 11.7 |
| 20–34 | 78.4 | 75.0 | 80.5 | 73.4 | 80.6 | 68.9 | 84.7 | 75.0 | 78.4 | 74.9 |
| >34 | 8.8 | 13.1 | 11.7 | 20.6 | 16.8 | 29.9 | 13.9 | 24.6 | 8.9 | 13.4 |
| Maternal race, % | ||||||||||
| White | 79.2 | 78.8 | 77.9 | 79.0 | 87.8 | 89.3 | 93.0 | 92.5 | 79.2 | 78.8 |
| Black | 16.3 | 15.2 | 18.9 | 16.7 | 10.1 | 7.4 | 5.3 | 4.5 | 16.4 | 15.3 |
| Other | 4.5 | 5.9 | 3.3 | 4.3 | 2.1 | 3.3 | 1.8 | 3.1 | 4.4 | 5.9 |
| Maternal education, % | ||||||||||
| ≤12 y | 57.7 | 52.7 | 53.5 | 43.2 | 37.6 | 25.2 | 30.5 | 24.0 | 57.6 | 52.5 |
| ≥16 y | 16.9 | 24.3 | 19.9 | 32.8 | 35.3 | 50.4 | 33.2 | 48.6 | 17.0 | 24.6 |
| Primipara, % | 41.6 | 41.0 | 20.6 | 22.0 | 18.7 | 21.8 | 18.1 | 16.1 | 41.1 | 40.4 |
| Married, % | 72.2 | 66.7 | 73.8 | 73.3 | 88.5 | 91.8 | 93.1 | 96.4 | 72.3 | 67.0 |
| Smoker, % | 12.4 | 9.3 | 11.7 | 7.6 | 6.2 | 2.4 | 3.0 | 1.2 | 12.4 | 9.3 |
| Adequate prenatal care, %c | 66.4 | 73.0 | 70.9 | 78.1 | 81.6 | 83.0 | 78.1 | 85.5 | 66.6 | 73.2 |
| No. of infants | 11 044 953 | 10 820 505 | 264 370 | 339 868 | 7946 | 19 185 | 626 | 1537 | 11 317 895 | 11 181 095 |
| Gestational age, mean (SD), wk | 39.2 (2.2) | 38.9 (2.2) | 36.0 (3.6) | 35.5 (3.4) | 32.8 (3.9) | 32.3 (3.7) | 30.9 (4.5) | 29.7 (3.7) | 39.1 (2.3) | 38.8 (2.3) |
| Birth weight, mean (SD), g | 3336 (549) | 3312 (540) | 2424 (649) | 2370 (626) | 1803 (592) | 1714 (555) | 1504 (720) | 1298 (535) | 3313 (570) | 3281 (570) |
| Birth weight categories, % | ||||||||||
| <1500 g | 1.0 | 1.0 | 8.9 | 9.3 | 27.4 | 32.4 | 55.3 | 69.2 | 1.2 | 1.3 |
| 1500–2499 g | 5.0 | 5.1 | 41.5 | 45.1 | 62.4 | 61.2 | 36.4 | 28.6 | 5.9 | 6.4 |
| ≥2500 g | 94.0 | 93.9 | 49.5 | 45.6 | 10.1 | 6.4 | 8.3 | 2.2 | 92.9 | 92.3 |
| No. of Infant deaths | 80 262 | 57 952 | 8902 | 8024 | 584 | 1005 | 75 | 148 | 89 823 | 67 129 |
| Neonatal (0–27 d) | 46 145 | 35 352 | 6989 | 6407 | 495 | 887 | 68 | 126 | 53 697 | 42 772 |
| Postneonatal (28–364 d) | 34 117 | 22 600 | 1913 | 1617 | 89 | 118 | 7 | 22 | 36 126 | 24 357 |
a
Quadruplets and quintuplets include 558 quadruplets and 68 quintuplets in 1989 –1991 and 1356 quadruplets and 181 quintuplets in 1999 –2001.
b
The numbers of mothers were calculated based on the number of infants per pregnancy, but because some multiple pregnancies may have included fetal deaths, this is only an estimate.
c
Adequacy of prenatal care is based on a modification of the Kessner criterion and includes month prenatal care began, number of prenatal visits, and length of gestation.