Factors Influencing the Likelihood of Instrumental.11

Original Research

Factors Influencing the Likelihood ofInstrumental Delivery Success

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

OBJECTIVE: To evaluate risk factors for unsuccessful

instrumental delivery when variability between individ-

ual obstetricians is taken into account.

METHODS: We conducted a retrospective cohort study

of attempted instrumental deliveries over a 5-year period

(20082012 inclusive) in a tertiary United Kingdom center.

To account for interobstetrician variability, we matched

unsuccessful deliveries (case group) with successful deliv-

eries (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 unsuc-

cessful (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 useof ventouse compared with forceps (OR 1.33; P,.05)were associated with unsuccessful outcome. When inter-

obstetrician variability was controlled for, instrument

selection and decision to rotate were no longer asso-

ciated with instrumental delivery success. More senior

obstetricians had higher rates of unsuccessful deliveries

(12% compared with 5%; P,.05) but were used to under-take 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 (OR1.03; 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 success-

ful instrumental delivery, whereas the influence of

instrument selection and rotational delivery appear to

be operator-dependent. Risk factors for lack of instru-

mental delivery success are distinct from risk factors for

requiring instrumental delivery, and these should not be

conflated in clinical practice.

(Obstet Gynecol 2014;123:796803)

DOI: 10.1097/AOG.0000000000000188

LEVEL OF EVIDENCE: II

Between 5% and 20% of newborns are delivered byinstrumental (operative vaginal) delivery in devel-oped countries.1 Overall, approximately 510% ofattempted instrumental deliveries will fail.2 Unsuc-cessful attempts are associated with a higher risk ofadverse maternal outcomes than that associated withproceeding directly to cesarean delivery, includingincreased rates of general anesthetic and wound infec-tion,3 as well as psychological trauma. Women whohave experienced a previous failed attempt are likelyto opt for an elective repeat cesarean delivery ratherthan another attempted vaginal birth.4 When instru-mental delivery is indicated because of fetal distress,neonatal outcomes also tend to be worse after anunsuccessful attempt.3

Established risk factors for requiring instrumentaldelivery include advanced maternal age,5 high bodymass index (BMI, calculated as weight (kg)/[height(m)]2), epidural analgesia, and high birth weight.6,7 Itis uncertain, however, whether or how these factorsinfluence the outcome of instrumental delivery. The

From the 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 PublicAffairs, and Red McCombs School of Business and Division of Statistics andScientific Computation, University of Texas at Austin, Austin, Texas.

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

Corresponding author: Catherine E. Aiken, MB/BChir, PhD, Department ofObstetrics and Gynaecology, University of Cambridge, NIHR CambridgeComprehensive Biomedical Research Centre, Cambridge CB2 2SW, UnitedKingdom; e-mail: [email protected].

Financial DisclosureThe authors did not report any potential conflicts of interest.

2014 by The American College of Obstetricians and Gynecologists. Publishedby Lippincott Williams & Wilkins.ISSN: 0029-7844/14

796 VOL. 123, NO. 4, APRIL 2014 OBSTETRICS & GYNECOLOGY

conflation of factors predicting the need for instru-mental delivery with factors predicting the likelihoodof success may be inappropriate and misleadingin intrapartum decision-making. The alternative toattempting instrumental delivery, however, is todirectly perform cesarean delivery during second-stage labor, which also has a high burden of morbid-ity.8 A recent Cochrane review concluded that there isno evidence from randomized trials to guide theobstetrician in the decision to attempt an instrumentaldelivery compared with proceeding directly to cesar-ean delivery.1 The aim of this study was to identifyrisk factors for unsuccessful instrumental delivery toaid the obstetrician in difficult decision-making.

MATERIALS AND METHODS

A cohort of 22,777 women with vertex-presenting,single, live born newborns at term (3742 completedweeks of gestation) aiming for vaginal delivery wasidentified over a 5-year period in a single tertiaryobstetrics center in the United Kingdom. Data regard-ing each womans pregnancy, labor, and delivery wererecorded by midwives soon after the birth and weresubsequently obtained from the hospitals Protos mater-nity data-recording system. The database is regularlyvalidated by a rolling program of audits whereby theoriginal case notes are checked against the informationrecorded in the database. Deliveries were classified ac-cording to the final mode of delivery (Fig. 1). Unsuc-cessful instrumental deliveries were defined as thoseduring which an instrument was applied to the fetalhead, but the eventual mode of delivery was cesareandelivery. The use of sequential instruments, wherebyany instrument was successful in delivering the new-born, was considered a successful delivery by the lastinstrument used. The rate of attempted instrumental

delivery did not vary significantly by year during thestudy period, nor did the rate of unsuccessful instru-mental delivery. The indications and procedures forinstrumental delivery in our center are as defined inthe operative vaginal delivery guidance from the RoyalCollege of Obstetricians and Gynaecologists.9

Characteristics of the maternalfetal dyad, includ-ing maternal age (at time of delivery), BMI (at firsttrimester prenatal booking), parity (before delivery),ethnicity, and the birth weight of the newborn, wereextracted from the hospital database. Birth weight wasrecorded to the nearest gram. The following variablesrelated to the delivery attempt were also noted:whether epidural analgesia was used before the deliv-ery attempt; the length of time between diagnosis ofsecond-stage labor and the time of delivery (time fullydilated); and the instrument selected. Gestational agewas recorded to the nearest week. Only cases of birthoccurring within the interval of 3742 weeks of com-pleted gestation were included. No adjustment wasmade for newborns found to be small or large forgestational age. No record of the station of the pre-senting part was available within our dataset. How-ever, to our knowledge, no delivery was performedwhen the presenting part was above the level of theischial spines, as recommended by Royal College ofObstetricians and Gynaecologists guidelines.9

The seniority of the obstetrician attempting deliv-ery was also recorded and classified into four types.Type 1 obstetricians were doctors who have finishedmedical school within the past 4 years; this groupconducted only 70 deliveries under supervision dur-ing the study period. Type 2 obstetricians are doctorswith 35 years of obstetric training. Type 3 obstetri-cians are senior trainees with 510 years of obstetrictraining. Type 4 obstetricians typically have more

Fig. 1. Outcomes of all deliverieswithin the study period.

Aiken. Success in Instrumental Delivery.Obstet Gynecol 2014.

VOL. 123, NO. 4, APRIL 2014 Aiken et al Success in Instrumental Delivery 797

than 10 years of clinical obstetric experience. Ourstudy was conducted in a unit where two obstetriciansare available to perform instrumental deliveries orcesarean deliveries during a 12-hour shift. The firstof these obstetricians is typically a type 2 obstetrician,and the second is a doctor with more than 5 years ofobstetric traininga type 4 obstetrician during the dayor a type 3 obstetrician overnight. All of the seniorobstetricians (type 3 or 4) were willing to attempt fetalhead rotation when they considered this to be safe.The method of fetal head rotation varied betweendifferent obstetricians but included manual rotation,ventouse (using the Kiwi Omnicup or rotational orposterior metal cup), and Kielland forceps. The posi-tion of the fetal head is not available within our data-base, but the majority of newborns who were not inthe occipito-anterior position are most likely to haveundergone an attempt at rotation in accordance withstandard procedure. A small number may have beendelivered in the direct occipito-posterior position, butthis would be a highly unusual occurrence, and thesedata are not recorded.

In our statistical analyses, groupwise compar-isons were performed using either Student t testor the Mann-Whitney test for numerical data andPearson x2 test for categorical data. Several multi-variate regression models were also fit as describedherein. Findings were considered statistically signif-icant at an alpha level of 0.05. All data analysis wasconducted using the R statistical software packageversion 2.14.1.

Failed instrumental delivery was modeled usinglogistic regression with the following covariates: birthweight; maternal age; ethnicity; maternal BMI;seniority of obstetrician; parity; delivery during day-light hours; and use of epidural analgesia. Separateanalyses were performed for two cohorts: the fullcohort and a case-control subset. The full cohortcomprised all successful and unsuccessful instrumen-tal deliveries. The case-control subset comprised allunsuccessful instrumental deliveries (case group),together with only those successful deliveries thatoccurred within the same 12-hour shift as an unsuc-cessful delivery (control group). The goal of analyzingthe case-control subset separately was to account formultiple sources of unobservable variation specific toa delivery unit that cannot be readily modeled. Thisincludes the experience and clinical judgment ofa particular obstetrician, the workload of the unitduring a given shift, the clinician with overall respon-sibility for the unit, subtle variations in day shiftscompared with night shifts or weekends, and otherintangible environmental factors. The interobstetrician

variability within the data are also significantly reducedby this strategy, because a maximum of two obstetri-cians are available for deliveries within any 12-hourshift. Analysis of the case-control subset is importantfor testing the robustness of our conclusions becausedifferences among operators may account for signifi-cant variability in the full cohort.

A further consideration is that the more seniorobstetricians are likely to have performed moredifficult cases, thereby skewing the apparent successrates. To check the robustness of our findings, weperformed separate analyses stratified by obstetriciantype, examining the associations between failedinstrumental delivery and those predictors thatappeared significant in the full cohort model.

Given the influence of birth weight on the likeli-hood of success of instrumental delivery, we examinedwhether birth weight is predictable using only thosecovariates that are observable by the obstetricianbefore attempting instrumental delivery. This wasperformed using ordinary least squares, with predictorschosen using Bayesian information criterion.

As a final robustness check, we also used classifica-tion and regression trees10 to build nonlinear predictivemodels for failed instrumental delivery and for birthweight. Classification and regression trees allow us touncover nonlinear structure and interactions amongthe predictors, thereby relaxing the more stringent para-metric assumptions of linear and logistic regression.

Finally, we sought to identify any systematicdifferences between women who underwent anattempted instrumental delivery (regardless of theoutcome) compared with those who directly under-went cesarean delivery during the second stage oflabor. We therefore examined the associationsbetween first attempted mode of delivery and thecovariates included in the original logistic regressionanalyses of successful instrumental delivery.

No patient-identifiable data were accessed in thecourse of this research, which was performed as partof a provision of service study for the obstetricscenter. Institutional Review Board approval thereforewas not required.

RESULTS

Three thousand seven hundred ninety-eight instru-mental deliveries were attempted, representing 16.7%of all attempted vaginal deliveries. Two hundredforty-six (6.5%) attempts at instrumental delivery wereunsuccessful. The overall number of instrumentaldeliveries performed did not differ between day andnight shifts, nor did the rate of unsuccessful instru-mental deliveries change between days and nights.

798 Aiken et al Success in Instrumental Delivery OBSTETRICS & GYNECOLOGY

Characteristics of the maternalfetal dyad werecompared according to the outcome of attemptedinstrumental delivery (Table 1). Only gestational age(P,.01) and birth weight (P,.001) exhibited statisti-cally significant differences between the two groups.Characteristics of the delivery attempt were also com-pared according to outcome (Table 1). The followingstatistically significant differences between the groupsemerged: the instrumental selected (P,.05); need forrotation of the fetal head (P,.001); seniority of obste-trician (P,.001); epidural analgesia (P,.001); and timefully dilated (P,.001). Sequential instruments wereused in 14 cases of unsuccessful instrumental delivery

(0.36% of the study population); an attempt at forcepsdelivery was made after failed ventouse in 12 cases andthe sequence was reversed in two cases. Because therewere a small number of these cases, they were catego-rized according to the last instrument used.

Table 2 shows the results of the regression analy-sis for the full cohort. Unsuccessful instrumental deliv-ery is associated with increased birth weight (oddsratio [OR] 1.11; P,.001), longer time fully dilatedbefore instrumental delivery (OR 1.01; P,.001), needfor rotation of the fetal head (OR 1.52; P,.05), andthe use of ventouse rather than forceps (OR 1.33;P,.05).

Table 1. Characteristics of the MaternalFetal Dyad and the Delivery Attempt, for the Full Data Set andStratified By Outcome

Characteristic All Patients (N53,798)Successful InstrumentalDeliveries (n53,552)

Unsuccessful InstrumentalDeliveries (n5246)

Maternal age (y) 30.1 (1940) 30.1 (1940) 30.0 (1840)Maternal BMI (kg/m2) 25.0 (1836) 25.0 (1836) 25.2 (1940)Birth weight (g) 3,487 (2,6104,440) 3,460 (2,6004,430) 3,709 (2,9454,654)*Gestation (wk) 39.9 (3742) 39.9 (3742) 40.1 (3842)

EthnicityWhite 3,352 3,131 221Southeast Asian 210 197 13Black 43 41 2Chinese 59 58 1Other or unknown 134 125 9

Parity0 2,008 1,879 1301 1,545 1,438 1052 198 189 83 29 27 34 or more 18 19 0

Time fully dilated (min) 132.3 (12282) 128.8 (12275) 132.5 (32327)*Rotation required

Yes 365 317 48*No 3,433 3,235 198

Instrument usedForceps 2,212 2,076 136Ventouse 1572 1,476 96Both 14 0 14

EpiduralYes 2,338 2,173 165*No 1,146 1,076 70Unknown 314 303 11

Obstetrician type1 70 70 02 2,760 2,632 128*3 718 629 894 236 208 28Unknown 14 13 1

Data are mean (95% coverage interval) or n.Numeric data are summarized by the mean and a coverage interval (in parentheses) spanning the 2.597.5 percentiles. Associations that

meet the threshold for statistical significance (alpha level50.05) are shown in bold.* P,.001. P,.01.


Table 3 shows the results of the regression analy-sis for the case-control subset. Increased birth weight(P,.001) and longer time fully dilated (P,.001)remained statistically significant, even after account-ing for interobstetrician variability. The need for rota-tion and the instrument used were no longersignificant at the 0.05 level.

Table 4 shows the results of using linear regres-sion to predict birth weight. Factors associated withhigher birth weight are gestational age (P,.001) andhigher parity (P,.01). Southeast Asian ethnicity isassociated with lower birth weight (P,.01). Afterrefining the model using stepwise selection, approxi-mately 22% of the variance in birth weight could beaccounted for. This figure is not an artifact of linearregression; when using classification and regressiontrees, a fully nonlinear method, only 24% of the var-iance in birth weight could be accounted for. Thissuggests that birth weight is difficult to predict accu-rately using information available at the time of deliv-ery (Fig. 2A).

Women who underwent cesarean delivery with-out a previous attempt at instrumental deliveryhad larger newborns (OR 1.07; P,.001), were older(OR 1.03; P,.01), and were more likely to have

Table 2. All Cases of Successful InstrumentalDelivery Compared With All Cases ofUnsuccessful Instrumental Delivery UsingMultivariate Analysis With a BinomialLogistic Regression Model

Variable Odds Ratio (95% CI)

Rotation (not required) ReferenceRotation (required) 1.52 (1.022.36)*Birth weight (per 100-g increase) 1.11 (1.081.15)

Time fully dilated 1.01 (1.001.01)

Parity 0.91 (0.751.24)Maternal age 1.01 (0.981.04)Day shift ReferenceNight shift 0.93 (0.751.23)Instrument (forceps) ReferenceInstrument (ventouse) 1.33 (1.011.77)*Ethnicity

White ReferenceBlack 1.06 (0.173.57)Southeast Asian 1.45 (0.742.58)Chinese 0.10 (0.0021.38)Other or unknown 1.30 (0.592.50)

No epidural ReferenceEpidural 1.23 (0.921.67)

CI, confidence interval.Model coefficients are expressed as odds ratios and 95% CIs.Associations that meet the threshold for statistical significance

(alpha level50.05) are shown in bold.* P,.05. P,.001.

Table 3. Multivariate Analysis Using a BinomialLogistic Regression Model of MatchedCases and Controls


Rotation (not required) ReferenceRotation (required) 2.24 (0.975.26)Birth weight (per 100-g increase) 1.14 (1.081.22)*Time fully dilated 1.01 (1.001.01)*Parity 0.87 (0.581.27)Maternal age 1.02 (0.971.07)Day shift ReferenceNight shift 1.24 (0.752.06)Instrument (forceps) ReferenceInstrument (ventouse) 0.90 (0.541.50)Ethnicity

White ReferenceBlack 0.73 (0.036.35)Southeast Asian 1.99 (0.695.57)Other/unknown 5.29 (1.2724.59)

No epidural ReferenceEpidural 1.20 (0.702.06)

CI, confidence interval.Associations that meet the threshold for statistical significance

(alpha-level50.05) are shown in bold.All cases of unsuccessful instrumental delivery are matched to

cases of successful instrumental delivery within the same shift,when such a case exists. When an unsuccessful instrumentaldelivery had no successful delivery within the same shift, it wasnot included in the analysis. When multiple successful deliv-eries occurred within the same shift as an unsuccessful delivery,all matches were included in the analysis.

Model coefficients are expressed as odds ratios and 95% CIs.* P,.001.

Table 4. Influence of Parameters Known to theObstetrician Before Instrumental DeliveryAttempt on Birth Weight


Gestational age 4.88 (4.355.48)*Ethnicity

White ReferenceBlack 0.72 (0.202.63)Southeast Asian 0.10 (0.050.18)

Chinese 0.47 (0.151.51)Other 0.55 (0.231.33)

Parity 1.37 (1.111.69)

Maternal BMI 0.10 (0.101.20)Maternal age 0.98 (0.961.01)

CI, confidence interval; BMI, body mass index.Associations that meet the threshold for statistical significance

(alpha level50.05) are shown in bold.Multivariate analysis was performed using a logistic regression

model.Model coefficients are expressed as odds ratios and 95% CIs.* P,.001. P,.01.


had epidural analgesia (OR 1.46; P,.001) (Table 5).Newborns delivered by direct cesarean delivery,however, were not as large as those who underwenta failed instrumental delivery (3,616 g compared with3,711 g; P,.01).

Greater seniority of the obstetrician appeared toadversely influence the chance of a successful instru-mental delivery; type 2 obstetricians had an overallfailure rate of 5% compared with 12% for type 3 ortype 4 obstetricians (P,.05). However, further analy-sis of the deliveries performed by each obstetriciantype demonstrated that the deliveries performed bytype 3 or type 4 (more experienced) obstetricians weremore likely to have higher birth weight (P,.05) and torequire rotation (P,.001). After adjustment for thesefactors, type 3 obstetricians were significantly morelikely to succeed at instrumental delivery than type2 obstetricians, their junior counterparts (Fig. 3).

There was no difference in the use of forceps com-pared with ventouse depending on seniority ofobstetrician.

Finally, the analysis of the case-control subsetidentified birth weight and time fully dilated as theonly significant predictors of failed instrumentaldelivery, regardless of whether logistic regression orclassification and regression trees was used. Wetherefore reperformed the logistic regression modelon the full cohort, first using only birth weight asa predictor and then using only time fully dilated asa predictor (Fig. 2). This allows us to estimate theoverall probability of success compared with the twomajor covariates (something that the case-controlanalysis cannot estimate properly). In Figure 2, theestimated probability of successful instrumental deliv-ery is plotted against time fully dilated (Fig. 2B) andbirth weight (Fig. 2C). In both panels, the models are

Fig. 2. A. Scatterplot and least-squares fit for birth weight compared with time fully dilated, stratified by gestational age.Estimated probability of successful instrumental delivery compared with time fully dilated (B) and birth weight (C), stratifiedby gestational age. The black line shows the logistic regression estimate; the grey area shows 95% confidence interval.

Aiken. Success in Instrumental Delivery. Obstet Gynecol 2014.


stratified by gestational age, demonstrating that thesame broad trends hold across 3742 weeks of gesta-tion. They show a clinically significant decline in thelikelihood of success for larger newborns and forthose with a long duration of being fully dilated.

DISCUSSION

We observed that increased birth weight andincreased duration of second-stage labor are stronglyassociated with lack of success in instrumental deliv-ery in the unmatched and case-control analyses. Useof ventouse rather than forceps and attempted rota-tion of the fetal head are associated with lack ofsuccess in the unmatched analysis only.

One possible interpretation of the associationsbetween instrument selection, rotation, and instrumentaldelivery outcome is that their influence may beoperator-dependent. It is recognized that fetal headmalposition in the second stage of labor is a risk factorfor adverse labor outcomes.11 However, rotation of thefetal head is considered a controversial procedure bymany obstetricians, despite data showing low compli-cation rates.12,13 Although rotational instrumental deliv-ery in our study had a higher rate of failure thannonrotational delivery, this was not the case for indi-vidual experienced operators, suggesting that moreextensive experience with operative vaginal deliverywould benefit trainee obstetricians. Previous studieshave concluded, as we do here in the full cohort anal-ysis, that overall forceps delivery is more likely toachieve successful vaginal delivery than ventouse14,15;however, there is also evidence that operator prefer-ence for a particular instrument can affect the deliveryoutcome.16

Although more experienced obstetricians had thehighest unadjusted rates of unsuccessful instrumentalattempts, this is likely to be because more difficultdeliveries are usually handled by more senior obste-tricians. After adjusting for birth weight and the needfor rotation, junior obstetrics trainees had the highestadjusted rates of unsuccessful instrumental delivery,indicating that increased training and experience areimperative.

Our data show that instrumental delivery is noless likely to be successful in older mothers. Despitethis, we found an increased likelihood of progressiondirectly to cesarean delivery in older mothers in thesecond stage of labor. This may reflect obstetricianuncertainty regarding the likelihood of success ofinstrumental delivery in older mothers, because nodata have previously been available to demonstratesuccess rates.17 It may also be considered less impor-tant to avoid cesarean delivery in older women, whoare less likely to have further pregnancies.

A small number of previous studies have exam-ined risk factors for failed instrumental delivery, yetnone has been able to control for interobstetricianvariability. A major strength of our study is its novelmethodologic approach, which reduces variation inindividual obstetrician skill, differential thresholds inabandoning instrumental delivery for cesarean deliv-ery, and technique-dependent variations, includingchoice of instrument and need for rotation of the fetalhead. Whereas our findings are in general agreementwith the current literature,15,1820 our study populationshowed several important differences from those pre-viously reported. In particular, our population had

Fig. 3. Likelihood of success in instrumental deliveryclassified by obstetrician type.

Aiken. Success in Instrumental Delivery. Obstet Gynecol 2014.

Table 5. Cases of Instrumental Delivery ComparedWith Cases of Direct Cesarean DeliveryDuring Second-Stage Labor When NoInstrument Was Applied


Birth weight (per 100-g increase) 1.07 (1.051.09)*Maternal age 1.03 (1.011.05)

EthnicityWhite ReferenceBlack 0.81 (0.242.03)Southeast Asian 1.34 (0.862.00)Chinese 0.93 (0.352.21)Other/unknown 0.88 (0.421.64)

Time at full dilation 0.1 (0.11.00)Maternal BMI 1.00 (0.11.00)Parity 1.08 (0.941.24)Obstetrician 1.11 (0.951.30)Delivery during daylight hours 0.86 (0.701.04)Epidural anesthesia 1.46 (1.181.81)*

CI, confidence interval; BMI, body mass index.Associations that meet the threshold for statistical significance

(alpha level50.05) are shown in bold.Multivariate analysis was performed using a binomial logistic

regression model.Model coefficients are expressed as odds ratios and 95% CIs.* P,.001 P,.01.


a higher rate of instrumental delivery (16.6%) com-pared with other studied populations (56%).15,18,20

The use of forceps was also much higher in our study(58.2% compared with 16.0%),15 and rotational deliverywas conducted within our study. This implies greaterexperience and willingness to perform instrumentaldelivery within our center. The cesarean delivery rateof all attempted vaginal deliveries in our populationwas 13.8% (including 10.3% performed during the firststage of labor) (Fig. 1). The main limitations of ourstudy include the difficulty in classifying deliverieswhen sequential instruments were used and the inabil-ity of our database to identify a small number of new-borns presenting in the occipito-posterior position whomay have been delivered by instrument without rota-tion. Additionally, it is possible that the longer durationof second-stage labor during unsuccessful instrumentaldeliveries may be partially explained by the extra timerequired to perform cesarean delivery, but we areunable to distinguish this possibility from a clinicaleffect of having a prolonged second stage of labor usingthe data available.

Experience from cohorts like ours with high ratesof instrumental delivery and low rates of intrapartumcesarean delivery is especially important in light ofcurrent concerns regarding increasing cesarean deliv-ery rates worldwide and the drive to reverse this trend.We demonstrate that once the need for instrumentaldelivery has been determined, the factors involved arereduced to a simple problem of mass and orientation toachieve delivery. Birth weight is difficult to estimatebefore delivery; however, it is the major determinant oflikelihood of success. Continued training in instrumen-tal delivery for obstetricians is invaluable, and ourstudy demonstrates significant improvement in successrates with increasing experience, ability to select theappropriate instrument, and ability to rotate the fetalhead. Future research could focus on better methods ofbirth weight prediction and on safe, effective trainingstrategies for resident obstetricians.

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Factors Influencing the Likelihood of Instrumental.11

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