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IMOP: randomised placebo controlled trial of outpatient cervical ripening with isosorbide mononitrate (IMN) prior to induction of labour – clinical trial with analyses of efficacy, cost effectiveness and acceptability

Shrikant Bollapragada1, Fiona Mackenzie2, John Norrie3, Stavros Petrou4, Margaret Reid5, Ian Greer1, Inass Osman1 and Jane E Norman1*

Author Affiliations

1 Division of Developmental Medicine, Glasgow Royal Infirmary, Queen Elizabeth Building, 10 Alexandra Parade, Glasgow, G31 2ER, UK

2 Princess Royal Maternity Hospital, Glasgow Royal Infirmary, 10 Alexandra Parade, Glasgow, G31 2ER, UK

3 Centre for Healthcare Randomised Trials (CHaRT), Health Services Research Unit, Aberdeen University, Applicant and Trial Statistician CHaRT, Polwarth Building, Foresterhill, Aberdeen AB25 2ZD, UK

4 National Perinatal Epidemiology Unit, Institute of Health Sciences, Old Road Headington, Oxford OX3 7LF, UK

5 Public Health, University of Glasgow, Public Health and Health Policy, Division of Community Based Sciences, University of Glasgow, 1 Lilybank Gardens, Glasgow, G12 8RZ, UK

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BMC Pregnancy and Childbirth 2006, 6:25  doi:10.1186/1471-2393-6-25

Published: 25 July 2006



There is increasing interest in carrying out pre-induction cervical ripening on an outpatient basis. However, there are concerns about the use of prostaglandins, the agents commonly used in hospital settings for this indication, because prostaglandins induce uterine contractions that may lead to fetal hypoxia. Indeed, in a recent study we demonstrated abnormalities in 9% of fetal heart rate tracings performed following prostaglandin induced cervical ripening at term. In contrast, we confirmed in the same study that isosorbide mononitrate (IMN) (administered on an inpatient basis) was both effective in inducing cervical ripening at term, and was associated with no associated fetal heart rate abnormalities.


The aim of this study is to determine whether IMN self administered by women on an outpatient basis improves the process of induction of labour. Specifically, we hypothesise that the use of outpatient IMN will result in a shorter inpatient stay before delivery, decreased costs to the health service and greater maternal satisfaction with ripening and induction of labour, compared with placebo treatment.

In the study described here (the "IMOP" study), women scheduled for induction of labour at term, and who require pre-induction cervical ripening will be randomised to self-administer at home either IMN 40 mg, or a placebo, each vaginally, at 48 hours, 32 hours and 16 hours before scheduled hospital admission.

After admission to hospital, treatment will revert to the usual induction of labour protocol. We will compare the primary outcomes of the elapsed time interval from hospital admission to vaginal delivery, the costs to the health service of induction of labour, and women's experience of induction of labour in the two groups.


This trial will provide evidence on the efficacy of outpatient IMN for pre-induction cervical ripening at term. We will study a formulation of IMN which is cheap and widely available. If the treatment is effective, acceptable to women, and cost effective, it could be implemented into obstetric practice worldwide.

Trial registration

The trial has been registered on the International Standard Randomised Controlled Trial Number Register (ISRCTN) and given the registration number ISRTN39772441.