| Author; Year of Publication | Preterm birth (General) | Medically indicated/Iatrogenic | Spontaneous |
| Al-Matary et al.; 2021 | 15.5 | NA | NA |
| Ayed et al.; 2020 | 26.6 | 1.2 | NA |
| Crovetto et al.; 2021 | 11.4 | NA | NA |
| Cruz-Melguizo et al.; 2021 | 11.1 | 47.7 | 38.9 |
| Haye et al.; 2021 | 16.5 | NA | NA |
| Khoury et al.; 2020 | 14.6 | NA | NA |
| Lokken et al.; 2021 | 9.7 | NA | NA |
| Martinez-Perez et al.; 2021 | 13.8 | 7.7 | 6.1 |
| Metz et al.; 2021 | 16.7 | 10.6 | 6.2 |
| Saccone et al.; 2020 | 26.3 | 80.0 | 20.0 |
| Taya et al.; 2020 | 18.5 | 8.8 | NA |
| Villar et al.; 2021 | 22.5 | 18.8 | 3.8 |
| Vousden et al.; 2021 | 18.3 | 14 | 4 |