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Factors Influencing the Likelihood of Instrumental Delivery Success

Aiken, Catherine E. MB/BChir, PhD; Aiken, Abigail R. MB/BChir, MPH; Brockelsby, Jeremy C. MBBS, PhD; Scott, James G. PhD

doi: 10.1097/AOG.0000000000000188
Contents: Original Research
Journal Club

OBJECTIVE: To evaluate risk factors for unsuccessful instrumental delivery when variability between individual obstetricians is taken into account.

METHODS: We conducted a retrospective cohort study of attempted instrumental deliveries over a 5-year period (2008–2012 inclusive) in a tertiary United Kingdom center. To account for interobstetrician variability, we matched unsuccessful deliveries (case group) with successful deliveries (control group) by the same operators. Multivariate logistic regression was used to compare successful and unsuccessful instrumental deliveries.

RESULTS: Three thousand seven hundred ninety-eight instrumental deliveries of vertex-presenting, single, term newborns were attempted, of which 246 were unsuccessful (6.5%). Increased birth weight (odds ratio [OR] 1.11; P<.001), second-stage labor duration (OR 1.01; P<.001), rotational delivery (OR 1.52; P<.05), and use of ventouse compared with forceps (OR 1.33; P<.05) were associated with unsuccessful outcome. When interobstetrician variability was controlled for, instrument selection and decision to rotate were no longer associated with instrumental delivery success. More senior obstetricians had higher rates of unsuccessful deliveries (12% compared with 5%; P<.05) but were used to undertake more complicated cases. Cesarean delivery during the second stage of labor without previous attempt at instrumental delivery was associated with higher birth weight (OR 1.07; P<.001), increased maternal age (OR 1.03; P<.01), and epidural analgesia (OR 1.46; P<.001).

CONCLUSION: Results suggest that birth weight and head position are the most important factors in successful instrumental delivery, whereas the influence of instrument selection and rotational delivery appear to be operator-dependent. Risk factors for lack of instrumental delivery success are distinct from risk factors for requiring instrumental delivery, and these should not be conflated in clinical practice.


After controlling for interobstetrician variability, higher birth weight and longer duration of second stage labor are associated with a higher likelihood of unsuccessful instrumental delivery.

Department of Obstetrics and Gynaecology, University of Cambridge, NIHR Cambridge Comprehensive Biomedical Research Centre, Cambridge, United Kingdom; and the Population Research Center and LBJ School of Public Affairs, and Red McCombs School of Business and Division of Statistics and Scientific Computation, University of Texas at Austin, Austin, Texas.

Corresponding author: Catherine E. Aiken, MB/BChir, PhD, Department of Obstetrics and Gynaecology, University of Cambridge, NIHR Cambridge Comprehensive Biomedical Research Centre, Cambridge CB2 2SW, United Kingdom; e-mail:

Dr. Aiken is supported by a Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Ruth L. Kirschstein National Research Service Award under grant number T32 HD007081-35 and by grant 5 R24 HD042849 awarded to the Population Research Center at the University of Texas at Austin by the NICHD. Dr. Scott is partially funded by a CAREER grant from the U.S. National Science Foundation (DMS-1255187).

Financial Disclosure The authors did not report any potential conflicts of interest.

© 2014 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.