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A hypothetical relationship between IVF and preterm birth mediated through multiple gestations.
| RR of preterm birth | |
|---|---|
| Total effect of IVF on preterm birth | 2.3 |
| Natural direct effect (independent of multiple gestations) | 1.2 |
| Natural indirect effect (attributed to multiple gestations) | 1.9 |
| Proportion mediated by multiple gestations* | 84% |
RR: risk ratio
*Calculated as RRNDE(RRNIE − 1)/(RRNDE × RRNIE − 1) (Vanderweele and Vansteelandt 2010).
