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Open Access Research article

Is it possible to estimate the minimal clinically important treatment effect needed to change practice in preterm birth prevention? Results of an obstetrician survey used to support the design of a trial

Sue Ross1234*, Jill Milne1, Shannon Dwinnell1, Selphee Tang12 and Stephen Wood12

Author Affiliations

1 Department of Obstetrics and Gynaecology, University of Calgary, Calgary T2N 2T9, Canada

2 Department of Community Health Sciences, University of Calgary, Calgary T2N 4Z6, Canada

3 Department of Family Medicine, University of Calgary, Calgary T2N 4N1, Canada

4 Department of Surgery, University of Calgary, Calgary T2N 2T9, Canada

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BMC Medical Research Methodology 2012, 12:31  doi:10.1186/1471-2288-12-31

Published: 19 March 2012



Sample sizes for obstetrical trials are often based on the opinion of investigators about clinically important effect size. We surveyed Canadian obstetricians to investigate clinically important effect sizes required before introducing new treatments into practice to prevent preterm birth.


Questionnaires were mailed to practicing obstetricians, asking the magnitude of pregnancy prolongation required to introduce treatments into practice. The three prophylactic treatments were of increasing invasiveness: vaginal progesterone, intramuscular progesterone, and cervical cerclage. We also asked about the perceived most relevant outcome measures for obstetrical trials and current obstetrical practice in preterm birth prevention.


544/1293(42.1%) completed questionnaires were received. The majority of respondents required one or two weeks' increase in length of gestation before introducing vaginal (372,77.1%), and intramuscular progesterone(354,67.9%). At least three weeks increase was required before introducing prophylactic cervical cerclage(326,62.8%). Clinicians who already used a treatment required a smaller difference before introducing it into practice. Decreasing neonatal morbidity was cited as the most important outcome for obstetrical trials (349,72.2%).


Obstetricians would require a larger increase in treatment effect before introducing more invasive treatments into practice. Although infant morbidity was perceived as a more important outcome, clinicians appeared willing to change practice on the basis of prolongation of pregnancy, a surrogate outcome. We found that there is not a single minimum clinically important treatment effect that will influence all practising clinicians: rather the effect size that will influence physicians is affected by the nature of the treatment, the reported outcome measure and the clinician's own current clinical practice.

Research design; Cross-sectional survey; Premature birth; Clinical trials as topic