Results
Our search strategy identified 978 studies. We selected 33 papers for
assessment of the full paper (Figure S1). We excluded 13 studies: 5
studies without CL measurement15-19, 2 studies with
the wrong population20,21, 2 studies considered
duplicate patient population22,23, and 1 study
included patients with a previous episode of threatened preterm
labor24. For three studies, there was concern about
trustworthiness of the data25,26,27. Two
studies25,26 had not shared data in an individual
participant data meta-analysis5, of which one study
had been retracted by the journal as it was not performed under the
appropriate ethics 25 . A third author did not respond
on our request for additional information26 (Table
S1). Two studies involving cerclage presented combined results for
singleton and twins only and were excluded because the authors could not
be contacted to assess only twins data28,29. Thus, we
included 20 studies (Table S2).