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Keywords:

  • Birthweight;
  • post-term;
  • pregnancy week;
  • risk factor;
  • shoulder dystocia

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References

Objective

To study whether pregnancy week at delivery is an independent risk factor for shoulder dystocia.

Design

Population study.

Setting

Medical Birth Registry of Norway.

Population

All vaginal deliveries of singleton offspring in cephalic presentation in Norway during 1967 through 2009 (n = 2 014 956).

Methods

The incidence of shoulder dystocia was calculated according to pregnancy week at delivery. The associations of pregnancy week at delivery with shoulder dystocia were estimated as crude and adjusted odds ratios using logistic regression analyses. We repeated the analyses in pregnancies with and without maternal diabetes.

Main outcome measures

Shoulder dystocia at delivery.

Results

The overall incidence of shoulder dystocia was 0.73% (n = 14 820), and the incidence increased by increasing pregnancy week at delivery. Birthweight was strongly associated with shoulder dystocia. After adjustment for birthweight, induction of labour, use of epidural analgesia at delivery, prolonged labour, forceps-assisted and vacuum-assisted delivery, parity, period of delivery and maternal age in multivariable analyses, the adjusted odds ratios for shoulder dystocia were 1.77 (1.42–2.20) for deliveries at 32–35 weeks of gestation, and 0.84 (0.79–0.88) at 42–43 weeks of gestation, using weeks 40–41 as the reference. In pregnancies affected by diabetes (n = 11 188), the incidence of shoulder dystocia was 3.95%, and after adjustment for birthweight the adjusted odds ratio for shoulder dystocia was 2.92 (95% CI 1.54–5.52) for deliveries at weeks 32–35 of gestation, and 0.91 (95% CI 0.50–1.66) at 42–43 weeks of gestation.

Conclusion

The risk of shoulder dystocia was associated with increased birthweight, diabetes, induction of labour, use of epidural analgesia at delivery, prolonged labour, forceps-assisted and vacuum-assisted delivery, parity and period of delivery but not with post-term delivery.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References

Shoulder dystocia is a rare obstetric emergency situation that may cause serious complications for both the mother and the child.[1-5] Shoulder dystocia is defined as a failure to spontaneously deliver the fetal shoulders after delivery of the head.[6] High offspring birthweight is the most important risk factor for shoulder dystocia.[7-10] Among other factors, prolonged labour, operative assisted vaginal delivery, a history of shoulder dystocia and induction of labour have also been associated with increased risk.[4, 11, 12]

It has been suggested that giving birth at pregnancy week 42 or later, regarded as post-term pregnancy, may increase the risk of shoulder dystocia.[6] Offspring birthweight increases with the duration of pregnancy, and for that reason alone shoulder dystocia may occur more often in these deliveries. Nonetheless, it is uncertain whether giving birth at 42 weeks of gestation or later is an independent risk factor for shoulder dystocia. Three previous studies have suggested that giving birth at 42 weeks of gestation or later is associated with increased risk,[13-15] but the results of another study suggested that that the association of shoulder dystocia with delivery in week 42 of gestation or later may be confounded by the higher birthweight in these offspring.[16] Maternal diabetes is a risk factor for shoulder dystocia, independent of birthweight.[4, 7, 17, 18] It is not known, however, whether women with diabetes are at particularly high risk of shoulder dystocia at 40 weeks of gestation or later.

In a population study of 2 014 956 vaginal deliveries of a singleton offspring in cephalic presentation we studied the association of weeks of gestation at delivery with the risk of shoulder dystocia. In the analysis we adjusted for offspring birthweight and other risk factors for shoulder dystocia. We also studied the association of pregnancy week at delivery with shoulder dystocia within strata of offspring birthweight and among pregnancies with and without maternal diabetes.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References

We performed a population-based registry study among all women giving birth in Norway during the period 1967–2009, using data from the Medical Birth Registry of Norway.[19] All deliveries are compulsorily reported to the Medical Birth Registry on standardised notification forms filled in by the attending midwife or physician shortly after delivery. A total of 2 521 086 deliveries after 16 weeks of gestation occurred during the study period. Vaginal deliveries of singleton offspring in cephalic presentation at pregnancy 32 weeks of gestation or after were included in our study population. We therefore excluded 67 024 non-singleton deliveries, 85 996 non-cephalic deliveries and 190 178 caesarean deliveries (8.1% of total). We also excluded 19 042 deliveries before 32 weeks of gestation because the risk of shoulder dystocia in these deliveries is negligible. Deliveries with missing information on pregnancy length (n = 107 253) or offspring birthweight (n = 2693) could not be included. A total of 26 006 pregnancies were reported to last longer than 43 weeks. As an unknown number of these deliveries were miscoded, we decided to exclude all of these pregnancies from the study sample. Hence, a total of 2 014 956 deliveries were included in the data analyses.

Information on shoulder dystocia at delivery was obtained by the standard notification form used by the Medical Birth Registry. From 1967 until 1999 information on shoulder dystocia was obtained by one open answer item related to complications at delivery. From 1999 one specified item ‘Complicated delivery of the shoulders’ with ‘yes or no’ as the response option, was included in the notification form.

Pregnancy week at delivery was our main explanatory variable and was divided into the following categories: 32–35, 36–37, 38–39, 40–41 or 42–43 weeks of gestation. Pregnancy week at delivery was estimated from the mother's reporting of the first day of her last menstrual period. After 1998, however, term date was estimated from a routine ultrasonographic examination at 17–19 weeks of gestation in 97% of the pregnancies in our study.

In the multivariable analyses, we also included the following known risk factors for shoulder dystocia as co-variates: offspring birthweight (in categories: <3000, 3000–3499, 3500–3999, 4000–4499, 4500–4999 or ≥5000 g), induction of labour (yes or no), use of epidural analgesia during labour (yes or no), prolonged labour—defined as labour lasting more than 24 hours (yes or no), forceps-assisted delivery (yes or no), vacuum-assisted delivery (yes or no), maternal diabetes—including type 1, type 2, gestational diabetes and unspecified diabetes (yes or no), parity (para 0, para 1 or para ≥2), maternal age (<20, 20–24, 25–29, 30–34 or ≥35 years old at delivery) and period (year) of delivery (1967–76, 1977–86, 1987–96 or 1997–2009).

We calculated the proportion of deliveries complicated by shoulder dystocia within the categories of weeks of gestation in the study sample as a whole and within the above described birthweight groups. Differences in proportions were tested using a chi-square test. The associations of each study factor with shoulder dystocia were estimated as crude and adjusted odds ratios (OR) with 95% confidence intervals (95% CI) using logistic regression analyses. We repeated the analyses in pregnancies with and without maternal diabetes. Finally, we assessed the interactions between pregnancy length at delivery and maternal diabetes in relation to the risk of shoulder dystocia by applying the Wald test. All statistical analyses were performed using the Statistical Package for Social Science for Windows version 15.0 (SPSS Inc., Chicago, IL, USA).

The study was approved by the Publishing Committee of the Medical Birth Registry of Norway and by the Norwegian Data Inspectorate.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References

In the study sample as a whole, 51.3% of the deliveries occurred at 40–41 weeks of gestation and 11.8% occurred at 42–43 weeks of gestation. Table 1 presents the distribution of study factors by weeks of gestation at delivery.

Table 1. Baseline characteristics of the study sample according to pregnancy week at delivery
 Pregnancy week at delivery
Total32–3536–3738–3940–4142–43
  1. The study sample includes all vaginal deliveries of a singleton offspring in ≥32 ≤ 43 weeks of gestation in cephalic presentation in Norway during 1967–2009, a total of 2 014 956 deliveries.

Total deliveries% ( n ) 2 014 9561.9 (39 110)5.4 (109 446)29.5 (594 608)51.3 (1 034 423)11.8 (237 366)
Mean birthweight (g) 354825703091344236753762
Induction of labour (%) 14.116.416.210.711.631.8
Epidural analgesia at delivery (%) 9.811.610.49.39.810.2
Prolonged labour >24 hours (%) 1.40.61.01.31.61.6
Forceps-assisted delivery (%) 2.32.21.81.72.43.5
Vacuum-assisted delivery (%) 5.02.63.83.95.37.1
Maternal diabetes (%) 0.61.41.60.80.40.2
Period (years)
1967–76100 (549 119)2.35.227.951.513.5
1977–86100 (412 451)1.84.927.252.713.5
1987–96100 (449 080)1.95.328.451.313.1
1997–2009100 (604 306)1.76.133.550.78.1
Parity
Para 0100 (815 957)2.35.227.951.013.5
Para 1100 (713 097)1.84.927.252.713.5
Para ≥2100 (485 902)1.95.328.451.313.1

Shoulder dystocia occurred in 0.73% (14 820/2 014 956) of all deliveries (Table 2). The proportion of deliveries complicated with shoulder dystocia, increased by increasing pregnancy length at delivery. In deliveries at 32–35 weeks of gestation, shoulder dystocia occurred in 0.22% of the deliveries, and in deliveries at 42–43 weeks of gestation the incidence was 0.97%. Using 40–41 weeks of gestation as the reference, the crude OR of shoulder dystocia was 0.27 (95% CI 0.22–0.33) for deliveries at <36 weeks of gestation and 1.17 (95% CI 1.11–1.22) for deliveries at 42–43 weeks of gestation (Table 2).

Table 2. Absolute risk, crude and adjusted odd ratios (OR) for shoulder dystocia according to pregnancy week at delivery, birthweight and other study factors
Total number (%)Total deliveriesShoulder dystociaCrude OR (95% CI)Adjusted ORa (95% CI)Adjusted ORb (95% CI)
2 014 95614 820 (0.73)
  1. a

    Adjustment made for offspring birthweight only.

  2. b

    Adjustment made for offspring birthweight at time of delivery, induction of labour, use of epidural analgesia at delivery, prolonged labour, forceps-assisted delivery, vacuum-assisted delivery, parity, period of delivery and maternal age.

Weeks gestation at delivery
32–3539 11087 (0.22)0.27 (0.22–0.33)1.68 (1.35–2.10)1.77 (1.42–2.20)
36–37109 446455 (0.42)0.50 (0.45–0.55)1.92 (1.74–2.12)1.76 (1.59–1.95)
38–39594 6063404 (0.57)0.69 (0.66–0.73)1.29 (1.24–1.34)1.28 (1.23–1.33)
40–411 034 4238582 (0.83)1.01.01.0
42–43237 3662292 (0.97)1.17 (1.11–1.22)0.88 (0.84–0.92)0.84 (0.79–0.88)
Birthweight (g)
<3000242 852121 (0.05)0.42 (0.34–0.51)0.36 (0.29–0.43)0.37 (0.31–0.46)
3000–3499644 216770 (0.12)1.01.01.0
3500–3999738 0373 224 (0.44)3.67 (3.39–3.97)3.92 (3.63–4.25)3.74 (3.45–4.04)
4000–4499318 8115740 (1.80)15.32 (14.21–16.50)17.10 (15.84–18.46)15.47 (14.30–16.70)
4500–499963 1903753 (5.94)52.77 (48.81–57.05)60.33 (55.73–65.32)52.44 (48.37–56.84)
≥500078591212 (15.45)152.58 (138.96–167.53)176.52 (160.52–194.10)156.99 (142.41–173.06)
Induction of labour 283 8093155 (1.11)1.65 (81.59–1.72) 1.20 (1.15–1.26)
Epidural analgesia 196 9632854 (1.45)2.21 (2.13–3.31) 1.18 (1.12–1.23)
Prolonged labour >24 hours 29 1431074 (3.69)5.49 (5.15–5.85) 1.39 (1.28–1.51)
Forceps-assisted delivery 46 134490 (1.06)1.46 (1.34–1.60) 1.14 (1.04–1.26)
Vacuum-assisted delivery 100 5252755 (2.74)4.44 (4.26–4.63) 2.86 (2.71–3.02)
Maternal diabetes 11 188442 (3.95)5.69 (5.17–6.27) 2.23 (2.0–2.48)
Parity
Para 0815 9574975 (0.61)1.0  
Para 1713 0975949 (0.83)1.37 (1.32–1.42) 1.16 (1.11–1.21)
Para ≥ 02485 9023896 (0.80)1.32 (1.26–1.37) 1.03 (0.95–1.08)
Period (years)
1967–76549 1191358 (0.25)1.0 1.0
1977–86412 4512349 (0.57)2.31 (2.16–2.47) 2.07 (1.93–2.16)
1987–96449 0804028 (0.90)3.65 (3.42–3.88) 2.97 (2.79–3.16)
1997–2009604 3067085 (1.17)4.79 (4.52–5.07) 3.40 (3.19–3.62)
Maternal age (years)
<20115 320483 (0.42)1.0 1.0
20–24533 0822928 (0.55)1.31 (1.19–1.44) 0.96 (0.87–1.06)
25–29693 1425171 (0.74)1.78 (1.62–1.96) 0.91 (0.83–1.01)
30–34460 3884284 (0.92)2.22 (2.02–2.44) 0.91 (0.82–1.0)
≥35201 8421954 (0.98)2.35 (2.13–2.60) 0.92 (0.83–1.03)
Missing34    

There was a strong positive association of offspring birthweight with the risk of shoulder dystocia. In pregnancies with offspring birthweight 3000–3499 g (reference), 0.12% of the deliveries were complicated by shoulder dystocia, and if the birthweight was 5000 g or more, shoulder dystocia occurred in 15.45% of the deliveries (crude OR, 152.7; 95% CI 139.04–167.53).

Adjustment for birthweight reversed the association of pregnancy week at delivery with shoulder dystocia. Hence, the adjusted OR for shoulder dystocia was 1.68 (95% CI 1.35–2.10) for deliveries at 32–35 weeks of gestation compared with deliveries at 40–41 weeks of gestation. For deliveries at 36–37 weeks of gestation the adjusted OR was 1.92 (95% CI 1.74–2.12), at 38–39 weeks of gestation the adjusted OR was 1.29 (95% CI 1.24–1.34), and the adjusted OR was 0.88 (95% CI 0.84–0.92) in deliveries at 42–43 weeks of gestation. Additional adjustment for the other study factors did not substantially alter these estimates (Table 2 and Figure 1).

image

Figure 1. Crude and adjusted odds ratios of shoulder dystocia according to pregnancy week at delivery.

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In addition to birthweight and pregnancy week at delivery, the following other factors were associated with increased risk of shoulder dystocia: maternal diabetes, vacuum-assisted delivery, prolonged labour, delivery in the last part of the study period, induction of labour, use of epidural analgesia at delivery and forceps-assisted delivery. Maternal age was not associated with the risk of shoulder dystocia (Table 2).

The consistently lower risk of shoulder dystocia with increasing pregnancy week at delivery was present within all strata of offspring birthweight, and showed a statistically significant trend in all strata except in deliveries of offspring <3000 g (Table 3, Figure 2).

Table 3. Absolute risk (%) for shoulder dystocia with 95% confidence intervals, according to pregnancy weeks at delivery within birthweight groups
Birthweight (g)Weeks of gestation at deliveryShoulder dystocia (%)95% CIShoulder dystocia (n)Total deliveries (n)P-valuea
  1. a

    Chi-square test: Linear-by-Linear Association.

<300032–350.050.03–0.031429 1950.155
36–370.060.04–0.082846 822 
38–390.060.04–0.075190 574 
40–410.040.02–0.052564 065 
42–430.02–0.01 to 0.05312 196 
Total0.050.04–0.06121242 852 
3000–349932–350.200.09–0.311260440.000
36–370.210.16–0.258641 386 
38–390.120.11–0.14298239 686 
40–410.110.10–0.12320299 938 
42–430.090.06–0.125457 162 
Total0.120.11–0.13770644 216 
3500–399932–350.890.33–1.232629160.000
36–370.680.61–0.7511416 773 
38–390.510.48–0.541016198 760 
40–410.410.39–0.431745424 942 
42–430.340.28–0.3732394 646 
Total0.440.43–0.453224738 037 
4000–449932–352.731.61–3.85228060.000
36–373.502.92–4.091323 767 
38–392.232.11–2.35126656 830 
40–411.721.66–1.773447200 684 
42–431.541.42–1.6487356 724 
Total1.801.78–1.855740318 811 
4500–499932–357.973.29–12.46111380.000
36–3711.388.87–13.8970615 
38–397.506.98–8.145977897 
40–415.825.60–6.05233440 072 
42–435.124.76–5.4874114 468 
Total5.945.76–6.12375363 190 
≥500032–3518.18–22.87 to 40.972110.000
36–3730.020.0–40.02586 
38–3921.6717.53–23.36176861 
40–4115.3013.99–16.137114722 
42–4314.2912.74–15.832982170 
Total15.4415.23–16.2312127850 
image

Figure 2. Incidence of shoulder dystocia according to categories of offspring birthweight (g) and pregnancy week at delivery.

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In pregnancies involving maternal diabetes, shoulder dystocia occurred in 3.95% (442/11 188) of the deliveries. In pregnancies involving maternal diabetes with delivery at 32–35 weeks of gestation, shoulder dystocia occurred in 2.12% (crude OR 0.66; 95% CI 0.36–1.19) compared with 3.31% in deliveries at 42–43 weeks of gestation (crude OR 1.04; 95% CI 0.58–1.85) (Table 4). Also in pregnancies involving diabetes, the OR for shoulder dystocia was reversed after adjustment for offspring birthweight. The adjusted OR for shoulder dystocia was 2.92 (95% CI 1.54–5.52) in deliveries at 32–35 weeks of gestation and 0.91 (95% CI 0.50–1.66) in deliveries at 42–43 weeks of gestation, using deliveries at 40–41 weeks of gestation as the reference.

Table 4. Absolute risks (%), crude and adjusted odd ratios (OR) for shoulder dystocia according to pregnancy week at delivery in pregnancies with maternal diabetes and in pregnancies without maternal diabetes
 Total deliveriesShoulder dystociaCrude OR (95% CI)Adjusted ORa (95% CI)
  1. a

    Adjustment made for offspring birthweight only.

Pregnancies with diabetes
Total, n (%) 11 188442 (3.95)  
Pregnancy week at delivery
32–3556712 (2.12)0.66 (0.36–1.19)2.92 (1.54–5.52)
36–37 173689 (5.13)1.64 (1.24–2.17)2.65 (1.97–3.56)
38–394738208 (4.39)1.39 (1.11–1.75)1.70 (1.37–2.20)
40–413754120 (3.20)1.01.0
42–4339313 (3.31)1.04 (0.58–1.85)0.91 (0.50–1.66)
Birthweight (g)     
<300011822 (0.17)0.31 (0.7–1.36)0.48 (0.56–1.09)
3000–3499276515 (0.54)1.01.0
3500–3999389087 (2.24)4.19 (2.42–7.27)4.66 (2.68–8.02)
4000–44992432170 (6.99)13.78 (8.10–23.42)15.94 (9.34–27.20)
4500–4999771121 (15.69)34.13 (19.82–58.71)39.76 (22.99–68.74)
≥ 500014847 (31.76)85.31 (46.16–157.67)102.71 (55.15–191.20)
Pregnancies without diabetes
Total number (%) 2 003 76814 378 (0.72)  
Pregnancy week at delivery
32–3538 54375 (0.19)0.24 (0.19–0.30)1.48 (1.17–1.90)
36–37107 713366 (0.34)0.41 (0.37–0.46)1.64 (1.47–1.83)
38–39589 8703196 (0.54)0.66 (0.66–0.69)1.24 (1.19–1.29)
40–411 030 6698462 (0.82)1.01.0
42–43236 9732279 (0.96)1.17 (1.12–1.23)0.88 (0.34–0.92)
Birthweight (g)     
<3000241 670119 (0.05)0.42 (0.34–0.51)0.37 (0.31–0.45)
3000–3499641 451755 (0.12)1.01.0
3500–3999734 1473137 (0.43)3.64 (3.36–3.94)3.84 (3.55–4.16)
4000–4499316 3795570 (1.76)15.21 (14.09–16.41)16.61 (15.38–17.94)
4500–499962 4193632 (5.82)52.43 (48.45–56.73)58.51 (54.00–63.41)
≥ 500077021162 (15.09)151.24 (137.56–166.27)170.73 (155.06–187.98)

In the study sample as a whole, there was significant interaction of pregnancy week at delivery and maternal diabetes in relation to risk of shoulder dystocia in multivariate analyses (P-value for interaction <0.05, Wald test).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References

Main findings

In this population-based study of more than 2 million vaginal deliveries, the crude risk of shoulder dystocia increased with increasing length of pregnancy. Birthweight increased by length of pregnancy, and birthweight was also strongly associated with shoulder dystocia. After adjustment for birthweight, the results showed a consistent reduction in risk with increasing length of pregnancy. Hence, at a given birthweight the risk of shoulder dystocia was higher in deliveries before 40 weeks of gestation than in deliveries at week 40 or later. This finding was particularly pronounced in pregnancies complicated by maternal diabetes. However, the confidence intervals of shoulder dystocia in women with diabetes were wide and overlapping across pregnancy weeks.

Strengths and weaknesses

Our study was population-based and included all deliveries in Norway during 1967–2009. A skewed selection to our study is therefore unlikely. All data were uniformly collected and derived from the Medical Birth Registry of Norway. The reporting of shoulder dystocia to the Medical Birth Registry is based on clinical judgement of the midwife or obstetrician in charge at delivery. We attempted to adjust for possible changes in the reporting of shoulder dystocia during our study period by including year of delivery in multivariable analyses, but the inverse association of length of pregnancy with shoulder dystocia remained nearly unchanged. Also, we performed separate analyses within time periods of delivery, and in both the periods 1967–76 and 1997–2009, the risk of shoulder dystocia decreased by increasing pregnancy week, after adjustment for birthweight (data not shown). Hence, possible changes in reporting of shoulder dystocia are not likely to have biased our estimates.

In our study, deliveries before pregnancy week 40 were more often performed by caesarean section than deliveries in week 40 or later (data not shown). As shoulder dystocia is not present in caesarean deliveries, a certain proportion of shoulder dystocia cases are therefore prevented, particularly in deliveries before 40 weeks of gestation. Therefore, the increased risk of shoulder dystocia associated with deliveries before 40 weeks of gestation that we found, is likely to represent underestimates.

Induction of labour was associated with shoulder dystocia, and induction of labour was more prevalent in deliveries at 42–43 weeks of gestation (31.8%) than at 32–36 weeks (16.4%). However, adjustment for induction of labour only slightly attenuated our estimates. In a separate analysis of spontaneous deliveries only, the results were essentially the same as for the study sample as a whole (data not shown).

Maternal body mass index and ethnic background of the mother are factors that may have confounded our results. Information on body mass index and ethnicity is not available in the Medical Birth Registry of Norway. Body mass index and the proportion of pregnant women with non-Norwegian background have increased during our study period.[20, 21] Adjustment for year of delivery, however, did not change our estimates, suggesting that our results are not confounded by maternal body mass index or ethnic background.

The offspring has an independent role in both the onset and progress of labour and the body configuration of the offspring may influence delivery.[22-24] It has been suggested that offspring with a large abdominal circumference relative to the head circumference have increased risk of shoulder dystocia.[24] During weeks 37–40 of gestation the growth of the offspring represents mainly increase in body fat. Therefore, it is unlikely that a large abdominal circumference relative to the head circumference provides an explanation for the increased risk of shoulder dystocia in preterm deliveries. It has also been suggested that the body configuration of offspring in pregnancies involving diabetes in particular increases the risk of shoulder dystocia.[24] We found an increased risk of shoulder dystocia associated with diabetes; however, in both pregnancies with and without diabetes there was a decreasing risk of shoulder dystocia by weeks of gestation at delivery after adjustment for birthweight.

To our knowledge, the association of pregnancy length at delivery with shoulder dystocia has been addressed in four previous studies only, and the results are inconsistent.[13-16] In three of these studies, the results suggested that deliveries in week 42 of gestation or later, increased the risk for shoulder dystocia. These studies, however, had limited statistical power to estimate associations of length of pregnancy and risk of shoulder dystocia with adjustment made for offspring birthweight and only crude associations were reported.[13-15] Results from another study, including 445 241 deliveries, suggested that the increased risk for shoulder dystocia in deliveries in week 42 or later could be explained by the higher offspring birthweight in these deliveries.[16] These findings are consistent with our results, but that study did not report the risk of shoulder dystocia associated with deliveries before 37 weeks of gestation.

Interpretations

The higher risk of shoulder dystocia at a given birthweight in deliveries before week 40, as compared with deliveries in week 40 of gestation or later is not easily explained. A successful delivery depends on a complex interplay between the uterus, the pelvis (the pelvic girdle and the pelvic floor muscles) and the offspring. During pregnancy, physiological changes take place to promote a favourable delivery. These changes include activation of the muscles of the uterus and the cervix.[22, 25] Also changes in the elasticity and collagen structure of the pelvic floor muscles occur during pregnancy.[26] The increasing serum concentration of prostaglandins and other hormones in maternal circulation may be important for such changes to occur.

The physiological changes that occur during pregnancy to prime the pregnant women and the offspring for delivery are likely to be completed by the estimated term.[27] Hence, the risk for delivery complications, such as shoulder dystocia, may be increased in deliveries before term. We found that the increased risk for shoulder dystocia in deliveries before 40 weeks of gestation was particularly pronounced in pregnancies involving diabetes. This finding suggests that hormonal factors may play a role in the priming of the pregnant body for delivery.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References

In our study of all vaginal deliveries in Norway in the period 1967–2009, we found that women giving birth to a large offspring before 40 weeks of gestation seem to be at higher risk of shoulder dystocia than women giving birth to a large offspring at 40 weeks of gestation or later. Such knowledge may be important for providing adequate obstetric care in women who expect a large offspring.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References

We are indebted to personnel at obstetrics wards in Norway and to the Medical Birth Registry of Norway for making data available for research.

Contribution to authorship

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References

EAØ wrote the manuscript, conceived the original idea and performed the data analyses. AE and LJV wrote the manuscript and contributed significantly to the interpretation of the results. All authors have approved the final version of the manuscript.

Details of ethics approval

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References

The study was approved by the Publishing Committee of the Medical Birth Registry of Norway and by the Norwegian Data Inspectorate.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure of interests
  10. Contribution to authorship
  11. Details of ethics approval
  12. Funding
  13. References
Commentary on ‘Pregnancy week at delivery and the risk of shoulder dystocia: a population study of 2 014 956 deliveries’

The article by Øverland et al. presents a population-based analysis of 33 years of data from Norway. The authors sought to determine if week of gestation (for births ≥32 weeks) was an independent risk factor for shoulder dystocia. After adjusting for birthweight and other obstetric factors with multivariable analysis, the authors found that infants born at 32–35 weeks had higher odds of shoulder dystocia than infants born at term or post-term. The authors correctly conclude that term and post-term gestational age per se is not positively associated with shoulder dystocia.

It is unclear why the authors generated the hypothesis that advanced gestational age is an independent risk factor for shoulder dystocia. Of course fetal weight and gestational age are highly correlated (as demonstrated by these authors and many others) and it seems obvious that this correlation accounts for the observation that shoulder dystocia increases with each week of gestation. Moreover the large baby at earlier gestation is more likely to be large as a result of pathological excess growth and to have a different body composition than a large fetus at term. To place the information contained in this article in a proper perspective, it is important to note that according to the authors' analysis gestational age at delivery of 32–35 weeks has an adjusted odds ratio of 1.68 for shoulder dystocia, as opposed to 0.88 for weeks 42–43, using the 40–41-week interval as the reference. However, birthweight remains by far the most important determinant of the risk of shoulder dystocia, with adjusted odds ratios of 15, 52 and 157 for birthweights of 4–4.5 kg, 4.5–5 kg and >5 kg, respectively, compared with newborns of 3–3.5 kg. Finally, because most large babies are born at later gestations, the contribution of large preterm fetuses to the population burden of shoulder dystocia is small.

Shoulder dystocia remains an enigmatic event in modern obstetrics; accurate prediction and prevention remain elusive (ACOG Practice Bulletin No. 40, Washington: ACOG, 2002; RCOG Green-top guideline No. 42, London: RCOG, 2012).

Disclosure of interests

We declare no conflicts of interest. The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

  • WM Callaghana & F Prefumob

  • aDivision of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, GA, USA

  • bDepartment of Obstetrics and Gynaecology, Maternal–Fetal Medicine Unit, University of Brescia, Brescia, Italy