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Extreme Obesity in Pregnancy in the United Kingdom

Knight, Marian MPH, DPhil; Kurinczuk, Jennifer J. MBChB, MD; Spark, Patsy BSc; Brocklehurst, Peter MBChB, MSc

doi: 10.1097/AOG.0b013e3181da8f09
Original Research

OBJECTIVE: To estimate the prevalence and describe on a national basis the management and outcomes of extreme obesity (body mass index [BMI] 50 kg/m2 or higher) in pregnancy in the United Kingdom.

METHODS: A national cohort study was undertaken. Extremely obese women (BMI 50 or higher) and a comparison cohort were identified using the UK Obstetric Surveillance System. Prevalence estimates were calculated with 95% confidence intervals (CIs). Data for obese and comparison women were compared using the χ2 test or the Wilcoxon rank sum test. Odds ratios were estimated throughout using unconditional logistic regression.

RESULTS: There were 665 women with extreme obesity in an estimated 764,387 women delivering, representing an estimated prevalence of 8.7 cases per 10,000 deliveries (95% CI 8.1–9.4). Extremely obese women had more complications than did comparison women, including preeclampsia (9% of women in the case group, 2% of those in the comparison group), gestational diabetes (11% of women in the case group, 2% of those in the comparison group), preterm delivery (10% of women in the case group, 7% of those in the comparison group), general anesthesia (6% of women in the case group, 1% of those in the comparison group), and intensive care unit admission (3% of women in the case group, 1% of those in the comparison group). Fifty percent of extremely obese women had cesarean deliveries compared with 22% of women in the comparison group. No women died.

CONCLUSION: Nearly 1 in every 1,000 women giving birth in the United Kingdom is extremely obese. These women have increased risks of poor outcomes. Basic equipment was not universally available for the care of these women, and this, together with the increase in prevalence of obesity, has important implications for maternity service provision. There is an urgent need to address prepregnancy care and weight management programs to prevent this increase in prevalence, as well as to ensure that appropriate services are in place to reduce the inequalities in pregnancy outcomes for these women.


One in every 1,100 women delivering in the United Kingdom is extremely obese; risks of poor outcomes are increased, and basic equipment is not universally available.

From the National Perinatal Epidemiology Unit, University of Oxford, Oxford, United Kingdom.

Dr. Knight is funded by the National Coordinating Centre for Research Capacity Development of the Department of Health.

The authors thank the UK Obstetric Surveillance System reporting clinicians, who notified subjects and completed the data collection forms.

This paper reports on an independent study that is funded by the Policy Research Programme in the Department of Health. The views expressed are not necessarily those of the Department.

Corresponding author: Marian Knight, Senior Clinical Research Fellow, National Perinatal Epidemiology Unit, University of Oxford, Old Road Campus, Oxford, United Kingdom; e-mail:

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

Obesity is an important public health problem throughout the developed and the developing world.1 The prevalence of obesity is rising rapidly in the United Kingdom, as in other countries, including in women of reproductive age.2 The prevalence of morbid obesity (World Health Organization [WHO] class 3, body mass index [BMI] more than 40 kg/m2) in women aged 25–34 in the United Kingdom has increased from 1.0% in 1993 to 2.2% in 2007, and evidence suggests that not only is obesity per se increasing, but the degree of obesity among those who are already obese is also increasing.2 There are no UK national data on the prevalence of obesity in pregnant women, despite well-recognized risks of obesity to both mothers and babies3–11 and the fact that the offspring of obese women are at higher risk of obesity in childhood and beyond.12

Recent reports of the UK Confidential Enquiry into Maternal Deaths13,14 have highlighted obesity as a factor in increasing numbers of maternal deaths in the United Kingdom. Twenty-seven percent of the women who died between 2003 and 2005 were classified as obese (WHO class 1, BMI 30 or greater).14

The majority of current studies focus on women with moderate/class 1 obesity (BMI more than 30). The studies therefore include only a very few women who are extremely obese and have not specifically addressed the risks in the extremely obese group, in whom pregnancy complications could be more common. Because of the relatively small numbers of women with this degree of obesity, national studies are required to investigate this further. The aim of this national cohort study was to estimate the prevalence of extreme obesity (BMI of 50 or greater) among pregnant women in the United Kingdom; to describe their characteristics, management, and pregnancy outcomes; and to estimate the risk of adverse outcomes attributable to obesity. We also sought to identify any adverse outcomes related to inadequate provision of equipment or services for these women.

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A national pregnancy cohort study was undertaken. Extremely obese women were identified through the UK Obstetric Surveillance System monthly mailings between September 2007 and August 2008. The UK Obstetric Surveillance System's negative surveillance methods have been described in detail elsewhere.15

The cohort included any pregnant woman who, at any point in pregnancy, had a BMI of 50 or more. We did not restrict the entry criteria to include only women who had a measured BMI in the first trimester of pregnancy so that we did not exclude the potentially important group of extremely obese women who attended late for antenatal care and were therefore at possible risk of poorer outcomes.

Comparison women were defined as any woman whose BMI was below 50. Reporting clinicians were asked to provide information on one comparison woman, identified as the woman delivering immediately before the extremely obese woman in the same hospital.

Prevalence estimates with 95% confidence intervals (CIs) were calculated using the most recently available national maternity data (2007) as a proxy denominator for 2007 and 2008.16

Data for obese and comparison women were compared using the χ2 test or the Wilcoxon rank sum test as appropriate. Odds ratios (ORs) were estimated throughout using unconditional logistic regression. ORs were adjusted for factors in a core model if there was a preexisting hypothesis or evidence from existing literature to suggest that the factors were potential confounders of any relationship between obesity and the relevant outcomes, for example, maternal age, parity, socioeconomic status, ethnicity, and smoking status. Continuous variables were tested for departure from linearity by the addition of quadratic terms to the model, and potential interaction terms between each variable and every other variable in the model were included, with subsequent likelihood ratio testing on removal. P less than .05 was considered evidence for significant interaction or departure from linearity. All analyses were carried out using STATA 10 (StataCorp LP, College Station, TX).

Over the study period we anticipated identifying 300 extremely obese women and 300 comparison women with an estimated power of 80% to detect ORs between 1.7 and 2.5 at the 5% level of statistical significance, for an incidence range for the outcomes of between 5% and 25% in the comparison group. For example, a study of this size would be able to detect an OR of 1.7 or greater for an outcome that occurred in 25% of the comparison women, such as cesarean delivery.

The UK Obstetric Surveillance System general methodology (04/MRE02/45) and this study (06/MRE02/77) were approved by the London Research Ethics Committee.

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All 226 eligible UK hospitals participated in the UK Obstetric Surveillance System, representing coverage of 100% of all births in the United Kingdom. Data collection was complete for 92% of notified women with extreme obesity in pregnancy (Fig. 1). A total of 665 extremely obese pregnant women were identified through the UK Obstetric Surveillance System in an estimated 764,387 deliveries,16 a prevalence of 8.7 per 10,000 deliveries (95% CI 8.1–9.4). Insufficient further information was received about 31 comparison women, thus subsequent analyses were performed on 665 eligible extremely obese women and 634 comparison women.



The median BMI of obese women was 53 (range, 50–80) and of comparison women 25 (range, 16–48; Fig. 2). Basic demographic and pregnancy-related characteristics of extremely obese and comparison women are shown in Table 1. Obese women were older and significantly less likely to have managerial or professional occupations or to be from a black or other minority ethnic group than comparison women. They had significantly more preexisting medical conditions and were more likely to have had difficulty conceiving, more highly parous, and more likely to have had a previous cesarean delivery than the comparison group.



Table 1

Table 1

Forty-seven obese women (7%) first attended for antenatal care beyond 20 weeks of gestation, compared with 35 comparison women (6%; P=.26). Ninety-six percent of obese women (n=639) received care at the usual hospital for their place of residence, and only 7 (1%) were referred to a tertiary center for care because of underlying medical conditions, comparable with the proportions for comparison women (data not shown). Obese women were seen by significantly more nonmidwifery health professionals antenatally than comparison women (Table 2). Eighty obese women (12%) were given thromboprophylaxis with low-molecular-weight heparin antenatally, compared with eight women (1%) in the comparison group (P<.001). Postnatally, 443 obese women (67%) and 127 comparison women (20%) received low-molecular-weight thromboprophylaxis (P<.001); postnatal thromboprophylaxis was also used more commonly in obese women delivering vaginally (132 of 324 [41%] obese women compared with 14 of 493 [3%] comparison women, P<.001).

Table 2

Table 2

Appropriate weight capacity equipment was not available at delivery for all women (Fig. 3), although there were no specific reported incidents of adverse events associated with the use of inappropriate capacity equipment.



Two obese women (0.3%) were reported who had first-trimester miscarriages and a further four (0.6%) had a second-trimester loss; these women were therefore not included in the further analysis of pregnancy outcomes. Maternal pregnancy complications are shown in Table 3. Extremely obese women were at higher risk for gestational hypertensive disorders, gestational diabetes, preterm delivery (before 37 completed weeks of gestation), cesarean delivery, and admission to the intensive care unit. Fifty-seven percent of the 65 preterm deliveries in extremely obese women (n=37) were iatrogenic, that is, labor was induced or the woman delivered by cesarean because of maternal or fetal complications. Obese women were more likely to undergo induction of labor (adjusted OR 1.97; 95% CI 1.53–2.54), but less likely to labor (adjusted OR 0.38; 95% CI 0.28–0.50). The adjusted analysis revealed a significant interaction between ethnicity and obesity (P=.008) on the risk of gestational diabetes requiring insulin, demonstrating the highest odds in extremely obese white women (adjusted OR 15.7; 95% CI 4.75–51.8; Table 3), with no increase in risk in extremely obese ethnic minority women. An interaction between obesity and parity was also observed in the association with pregnancy-induced hypertension (P=.047); the risk in primiparous obese women (adjusted OR 7.35; 95% CI 3.07–17.6) was more than double that in multiparous obese women, although the risk remained significantly increased in multiparous women (adjusted OR 3.19; 95% CI 1.60–6.37). We did not identify any other interactions but note that the study may have limited power to detect them.

Table 3

Table 3

Table 3

Table 3

Fifty percent of obese women (n=328) had a cesarean delivery (49% [n=160] electively and 51% [n=168] as an emergency), compared with 22% of comparison women (n=140) (adjusted OR 3.50; 95% CI 2.72–4.51). The most common indications for elective cesarean delivery were previous cesarean delivery (54%, n=86), abnormal fetal presentation (breech, transverse, or unstable lie [18%, n=28]), and maternal medical complications (9%, n=15). Obese women were six times more likely to have general anesthesia for delivery than comparison women (adjusted OR 6.35; 95% CI 2.63–15.3) and had significantly more problems with other anesthesia methods (Table 3). Seventy-three women (22% of those who underwent cesarean delivery) had a postoperative complication; the most frequent were wound infection (n=48, 66%) or wound dehiscence (n=10, 14%).

No women died in either group. Pregnancy outcomes are known for 652 of the 659 obese women with ongoing pregnancies in the third trimester. These women delivered a total of 671 infants, including 15 pairs of twins and 2 sets of triplets. Fifty-nine of 635 singleton infants (9%) born to obese women had birth weights of 4,500 g or more; 12 of 625 singleton infants of comparison women (2%) weighed 4,500 g or more at birth (OR 5.23; 95% CI 2.81–9.74). One quarter of the infants with birth weights of 4,500 g or more were born to extremely obese women with preexisting or gestational diabetes (14/59, 24%).

There were eight stillbirths and three early neonatal deaths among the infants born to obese women, representing a perinatal morbidity rate of 16 per 1,000 total births to extremely obese women (95% CI 8–29 per 1,000 total births). All the neonatal deaths occurred in infants born before 32 weeks of gestation; two of the stillbirths occurred in women with preexisting or gestational diabetes. There were five stillbirths and no early neonatal deaths among the infants born to comparison women, a perinatal mortality rate of 8 per 1,000 total births (95% CI 3–18 per 1,000 total births; OR 2.12; 95% CI 0.77–5.88).

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This study of an estimated 764,387 women giving birth has revealed on a national basis the extent of the problem of extreme obesity in pregnancy in the United Kingdom. More than 1 of every 1,200 women giving birth in the United Kingdom has a BMI of 50 or greater. Given current trends,2 the number of extremely obese women in pregnancy is likely to increase, thus emphasizing the importance of surveillance and evidence to address primary prevention and guide appropriate management and service provision. Extremely obese women are at higher risk for almost all the pregnancy complications we examined. These increased risks are not explained by differences in age, ethnic group, socioeconomic group, smoking, or parity. More extremely obese women entered pregnancy with known medical factors relating to adverse outcomes, including 5% with preexisting diabetes and 6% with hypertension. The proportion of extremely obese women with preexisting diabetes is higher than reported in other studies of women with more moderate obesity. For example, 2.2% of women in Nova Scotia weighing 120 kg or more in pregnancy had preexisting diabetes5; 0.8% of women with a BMI of 30 or greater delivering in London had preexisting diabetes.6

More than one in five extremely obese women developed a hypertensive disorder in pregnancy; nearly one in ten developed preeclampsia, a more than four-fold increase in odds over the comparison group. Other recent studies of women with more moderate degrees of obesity have also reported an increased risk of hypertensive disorders, but generally with a lower absolute and relative risk.6,17 Gestational diabetes was diagnosed in 11% of extremely obese women, and nearly 70% of these women required insulin. The odds of gestational diabetes in extremely obese women were seven times higher than in the comparison group. These figures are compatible with other studies examining women with morbid obesity,4,5,18 and higher than in studies examining women with more moderate obesity. We noted an ethnic difference in the risk of gestational diabetes requiring insulin, such that the increase in risk was limited to white women. There were no ethnic differences in the risks of gestational diabetes per se, thus our results do not support the suggestion of underdiagnosis, but may reflect differences either in the way ethnic minority women are treated for gestational diabetes or alternatively between ethnic groups in the responsiveness of gestational diabetes to measures such as diet modification.

Obesity is a recognized risk factor for shoulder dystocia,19 yet shoulder dystocia was uncommon in this extremely obese cohort and not significantly different from the rate observed in the comparison group, despite nearly 1 in 10 of the infants of obese mothers weighing 4,500 g or more at birth. The most likely explanation for this is the extremely high rate of cesarean delivery in the obese group (50%). A higher rate of cesarean delivery in obese women has been identified in recent systematic reviews,9,20 with a pooled estimate of 38% in the morbidly obese group. Thus our results suggest that the risk of cesarean delivery is even higher in extremely obese women than in less obese women. Further work is required to explore whether this is due to obstetrician choice or due to other factors related to obesity such as inefficient uterine activity.

The outcomes for infants of extremely obese women were also generally poorer than for the comparison group. Infants were more likely to be born preterm and were more likely to weigh 4,500 g or greater at birth. Perinatal mortality was nonsignificantly increased in the infants of extremely obese women, primarily due to a higher number of neonatal deaths. Other studies consistently report an increase in risk of both stillbirth21 and neonatal death10,19,22,23 associated with maternal obesity, with an approximate doubling of the risk in obese mothers. In our study, all the neonatal deaths occurred in infants born preterm. The observed associations between maternal obesity and preterm birth are complex, with some studies reporting that obesity is protective,6,24,25 others reporting increased risk only in certain subgroups,11 and further studies suggesting an independent increase in risk associated with maternal obesity.26 In this cohort study of women with extreme obesity, we observed a significant increase in the risk of preterm birth at less than 37 weeks of gestation, independent of age, parity, socioeconomic status, smoking, or ethnicity. We did not find a significant increase in risk of delivery at less than 32 weeks of gestation, possibly because of a relatively small number of events and a consequent lack of study power. These findings would lead us to suggest that some of the conflicting results of other studies of obesity and preterm birth may be due to variable inclusion of extremely obese women. Subfertility or problems with conception were reported in almost 1 in every 10 extremely obese women, and the relatively high multiple pregnancy rate suggests that some women conceived using assisted reproductive techniques. It is important that the risks of pregnancy complications should be discussed before assisting extremely obese women to conceive.

This study shows the implications of the obesity epidemic for maternity services. Extremely obese pregnant women received care from a wider range of health professionals than comparison women; they had more complex pregnancies and more interventions, and were more likely to give birth to preterm and high birth weight infants, both groups susceptible to long-term health problems. Although we did not document any instances of lack of facilities contributing to poor outcomes for obese pregnant women, basic equipment to enable health professionals to care appropriately, and safely, for these women was not universally available. Adverse outcomes for women and staff, as well as a basic lack of comfort, are likely to result if equipment provision remains inadequate. Similar themes have been identified in recent qualitative studies27 and reviews.28 In the context of an increasing birth rate and rising levels of obesity,2,16 this is an area that needs addressing urgently.

For pragmatic reasons, and to maintain a link with the notification of extremely obese women, we identified the comparison group as women delivering immediately before the extremely obese women, provided their BMI was less than 50. This comparison group represents the general population of women giving birth in the United Kingdom, and hence has a BMI range between 16 and 48. Because a large proportion of extremely obese women underwent cesarean delivery, the comparison group was also more likely to have delivered during the daytime than the general population, and may thus be a higher-risk cohort. It should therefore be noted that the use of this comparison group may actually underestimate the risks associated with extreme obesity.

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© 2010 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.