Table 1.
Description of the study population
| Singletons at birth | Twins at birth | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Reference group* |
Excess ET | Reference group* |
Excess ET | ||||||
| Excess FHB | Excess FHB | ||||||||
| Embryos transferred (ET) | 1 | 2 | ≥3 | >1 | 2 | 3 | ≥4 | >2 | |
| Fetal heartbeats (FHB) | 1 | 1 | 1 | ≥2 | 2 | 2 | 2 | ≥3 | |
| N, children | 23,753 | 38,019 | 14,464 | 5437 | 42,851 | 9008 | 2720 | 2183 | |
| Maternal age (%) | Mean years ± SD | 35.5 ± 5.1 | 35.4 ± 5.0 | 37.7 ± 4.2 | 36.7 ± 4.9 | 34.7 ± 5.2 | 36.3 ± 4.4 | 38.2 ± 4.1 | 36.6 ± 4.2 |
| 18–29 | 9.6 | 10.5 | 3.9 | 7.1 | 13.9 | 6.5 | 3.3 | 4.4 | |
| 30–34 | 36.0 | 35.1 | 17.5 | 27.0 | 40.2 | 25.2 | 14.6 | 25.0 | |
| 35–37 | 23.5 | 24.4 | 21.3 | 23.3 | 21.8 | 27.4 | 18.3 | 28.0 | |
| 38–40 | 14.8 | 16.1 | 30.6 | 22.6 | 11.1 | 27.2 | 32.3 | 26.5 | |
| 41–43 | 8.8 | 7.4 | 21.7 | 12.7 | 5.8 | 9.1 | 26.4 | 12.1 | |
| ≥44 | 7.3 | 6.5 | 5.1 | 7.4 | 7.2 | 4.7 | 5.1 | 3.9 | |
| Race of mother (%) | White | 78.7 | 79.8 | 81.1 | 80.8 | 80.8 | 80.9 | 78.8 | 80.9 |
| Black | 4.9 | 6.5 | 6.6 | 6.7 | 6.1 | 6.8 | 8.6 | 6.1 | |
| Asian | 13.8 | 10.6 | 9.6 | 9.9 | 9.8 | 9.5 | 9.9 | 11.7 | |
| Other/missing | 2.5 | 3.1 | 2.8 | 2.6 | 3.2 | 2.8 | 2.7 | 1.2 | |
| Ethnicity of mother (%) | Hispanic | 7.2 | 9.4 | 8.3 | 8.4 | 10.8 | 10.3 | 12.1 | 11.1 |
| Mother’s BMI (%) | 12–24 | 64.1 | 57.8 | 57.7 | 58.6 | 58.9 | 57.9 | 56.3 | 61.0 |
| 25–29 | 21.7 | 24.4 | 23.8 | 23.3 | 23.8 | 24.3 | 26.4 | 21.1 | |
| 30–59 | 14.3 | 17.8 | 18.4 | 18.1 | 17.3 | 17.7 | 17.3 | 17.9 | |
| Missing | 12.3 | 20.5 | 30.0 | 22.7 | 21.4 | 36.3 | 41.8 | 40.3 | |
| Hypertension** | % | 7.9 | 8.2 | 8.1 | 8.3 | 15.2 | 13.7 | 14.2 | 15.9 |
| Diabetes** | % | 7.8 | 7.6 | 8.3 | 7.9 | 9.0 | 9.3 | 10.4 | 9.9 |
| Infant male sex | % | 52.7 | 51.2 | 50.5 | 50.9 | 51.2 | 50.7 | 49.4 | 50.3 |
| Birthweight | Grams, mean (SD) | 3324 ± 586 | 3277 ± 611 | 3259 ± 613 | 3122 ± 681 | 2374 ± 595 | 2388 ± 589 | 2362 ± 588 | 2254 ± 616 |
| Major birth defect | Rate*** | 237.4 | 258.0 | 257.9 | 253.8 | 341.4 | 351.9 | 327.2 | 329.8 |
| Nonchromosomal | 228.6 | 241.5 | 232.3 | 235.4 | 324.8 | 321.9 | 286.8 | 311.5 | |
| Chromosomal | 8.8 | 16.6 | 25.6 | 18.4 | 16.6 | 30.0 | 40.4 | 18.3 | |
| Small-for-gestational age birthweight**** | % | 7.0 | 8.1 | 8.5 | 11.5 | 19.8 | 22.1 | 21.9 | 25.8 |
| Large-for-gestation birthweight**** | % | 10.1 | 9.8 | 10.0 | 7.8 | 1.6 | 1.7 | 0.9 | 1.7 |
| Low birthweight (LBW) (<2500 g) | % | 7.2 | 8.4 | 8.6 | 13.8 | 54.7 | 53.5 | 55.9 | 63.0 |
| Length of gestation (%) | Weeks, mean (SD) | 38.6 ± 2.1 | 38.4 ± 2.2 | 38.3 ± 2.2 | 38 ± 2.7 | 35.3 ± 3 | 35.4 ± 2.9 | 35.3 ± 2.9 | 34.9 ± 3 |
| <28 weeks | 0.6 | 0.8 | 0.7 | 1.6 | 3.3 | 2.8 | 3.0 | 3.0 | |
| 28–32 weeks | 1.4 | 1.7 | 1.7 | 2.8 | 10.3 | 9.7 | 9.8 | 15.1 | |
| 33–36 weeks | 8.0 | 8.9 | 9.5 | 10.9 | 45.9 | 44.1 | 46.1 | 44.1 | |
| ≥37 weeks | 90.1 | 88.6 | 88.1 | 84.8 | 40.5 | 43.4 | 41.1 | 37.7 | |
| LBW at term (≥37 weeks, <2500 g) | % | 2.2 | 2.5 | 2.6 | 4.3 | 23.9 | 24.9 | 25.0 | 28.7 |
*Reference group, ET=1 and FHB=1 for singletons at birth and ET=2 and FHB=2 for twins at birth
**Pregestational (chronic) or gestational
***Rate per 10,000 children
****Small-for-gestational age birthweight is defined as a birthweight Z-score ≤−1.28; large-for-gestation birthweight is defined as a birthweight Z-score ≥1.28