Review article: Maternal obesity in pregnancy: is it time for meaningful research to inform preventive and management strategies?


U Krishnamoorthy, 7 Hawthorn Road, Bamford, Rochdale Ol11 5JG, UK. Email


The growing epidemic of obesity in our society has become a major public health issue, with serious social and psychological consequences in addition to the physical health implications. Obesity has reached epidemic proportions globally with a similar rise in prevalence among women in the reproductive age group. This has critical consequences for fetal and maternal health in the antepartum, intrapartum and postpartum periods. The aims of this study were to summarise the implications of maternal obesity on maternal, fetal and neonatal health and to recommend good practice guidelines on the management of this problem. The authors highlight the need for good quality interventional research on maternal obesity while identifying avenues with potential scope for future research in this context.

Background on the ‘obesity epidemic’

There has been a dramatic rise in the worldwide prevalence of obesity, leading to a World Health Organization (WHO) declaration that obesity is a major killer disease of the millennium on par with HIV and malnutrition. The rising prevalence of obesity has created a ‘global paradox’ in that the two extremes of malnutrition and obesity can coexist, at the same time, in the same country.1 Although there is a higher rate of men in the overweight category, globally more women are in the obese category. The epidemic of obesity is equally threatening the developing countries and the developed world.2

Definition of obesity

A common and universally acknowledged definition of obesity is: ‘a condition characterised by excess of body fat frequently resulting in a significant impairment of health and longevity’.The most frequently used criteria for measuring obesity is an elevated body mass index (BMI) of more than or equal to 30.3 Others include an increased waist circumference, an increased waist to hip ratio or a body weight of more than 90 kg. For this review, in accordance with the 1990 Institute of medicine (IOM) guidelines, an elevated BMI of more than or equal to 30 was chosen and this definition is widely acknowledged in the WHO surveys and The Health survey for England.

Normal weight gain in pregnancy

Recommendations for maternal weight gain in pregnancy were controversial during the 20th century. In 1990, the IOM recommended weight gain ranges for pregnant women depending on whether the pre-pregnancy weight was in the underweight, normal, overweight or obese ranges3 (Table 1). A systematic review in 2000 by the Californian School of Public Health showed that pregnancy weight gain within the IOMs ranges is associated with the best outcome for both mothers and infants.4

Table 1.  Recommended total weight gain ranges for pregnant women by pre-pregnancy BMI
Pre-pregnancy BMIRecommended weight gain (kg)Recommended weight gain (lb)
Underweight, <19.812.5–1828–40
Normal, 19.8–24.911.5–1625–35
Overweight, 25–29.97–11.515–25
Obese, >29.9At least 6.8 (higher limit not specified)At least 15 (higher limit not specified)

Worldwide prevalence of adult obesity

It is estimated that more than 300 million adults worldwide are obese, more than 1 billion are overweight and a further 115 million suffer related problems ranging from premature death to reduced overall quality of life.2 In the USA, approximately 64.5% of the total population is classified as either overweight or obese, with morbid obesity affecting more than 9 million adult Americans.5 The rising prevalence of obesity worldwide and in Europe are evident from the published figures at the International Obesity Task Force data base.6 Even more alarming is the rise of obesity among children worldwide, which is likely to have a significant effect on the future prevalence of adult obesity.1

Prevalence of obesity in UK

The prevalence of obesity in the UK in 2002 was estimated as 23% in women and 22% in men.7 The Health Survey for England 2004 revealed a steady rise in the prevalence of obesity in women from 16.4% in 1993 to 23.8% in 2004 and prevalence in men to increase from 13.2 to 23.6% between 1993 and 2004.8 This presents an enormous contemporary public health issue, with serious physical, psychological and social effects on the population. Despite this, healthcare provision for obesity is patchy.

Obesity is associated with more than 30 000 deaths every year as well as accounting for 18 million sick days and 40 000 lost years of working life. It costs the NHS half a billion pounds on direct costs and a further £2 billion to the wider economy. A House of Commons Report on Obesity estimated the economic cost of obesity to the country ‘conservatively’ at £3.3–£3.7 billion/year.9

Prevalence of maternal obesity in UK

Obesity in pregnancy carries significant maternal risk. This was highlighted in the last report of Confidential Enquiries into Maternal and Child Health (CEMACH) where obesity was identified as a risk factor for maternal death following the finding that 35% of all mothers who died were obese, representing a disproportionate number of deaths associated with obesity in childbearing women.10

In 2001, a retrospective study of 287 213 pregnant women in London found that 27.5% were overweight and 10.9% obese.11 A similar study of 30 167 pregnancies in Manchester found that of those where BMI was recorded, 18.5% were obese.12

These two studies, in conjunction with the CEMACH data, highlight how common the problem of obesity during pregnancy is in the UK. In fact, obesity is now the most common clinical risk factor encountered in obstetric practice. There is increasing concern that the rise in obese teenagers reaching childbearing age will exacerbate this problem.10

More than half a million women give birth annually in England and Wales.10 The BMI details of all these women are not known, but the ‘Maternal Obesity and Pregnancy outcome’ project, undertaken as a collaborative effort by the North East Public Health Observatory, The Food and Nutrition Group at the University of Teeside and The Regional Maternity Survey Office, is now collecting the relevant data for investigation into the relationship between obesity and outcomes of pregnancy.13 The increasing numbers of obese pregnant women and potential health and economic implications are staggering.

Risks associated with maternal obesity

Fetal risks associated with maternal obesity

A case–control study in 2004 evaluated the risk of miscarriage in 1644 obese women compared with an age-matched control group of 3288 women with normal BMI. The study found that obesity was associated with an increased risk of first trimester and recurrent miscarriage.14

When implantation rates, continuing pregnancy rates or spontaneous miscarriage rates were compared by retrospective data analysis in an obese and normal-weight population in women attempting conception through oocyte donation, no difference was found in implantation rates or pregnancy outcome. This suggests that obesity does not exert a negative effect on endometrial receptivity.15

Since 1994, an increasing number of studies have established an association between maternal obesity and infant birth defects.16–21 Reported anomalies have included neural tube defects, such as anencephaly, anomalies of heart and intestinal tract, omphaloceles, orofacial clefts and multiple congenital anomalies of the central nervous system. The reasons for an association with maternal obesity are poorly understood, although one hypothesis suggests association with undetected type II diabetes in early pregnancy. Many authors acknowledge that there are other explanations that are yet to be identified.

The increased prevalence of neural tube defects in the offspring of pregnant obese population may relate to lower circulating levels of folate. Obese women may therefore benefit from a higher dose of preconception folate supplement.22 As the higher incidence of infant neural tube defects persisted in one study despite food fortification with folic acid, further studies are needed to investigate this.16

Obesity is well known to be associated with macrosomia, leading to potential adverse maternal outcomes from obstetric intervention (induction of labour, caesarean section) and adverse neonatal outcomes from shoulder dystocia (birth injuries such as nerve palsies).23–27 Maternal obesity has been associated with fetal distress and an increased risk of fetal meconium aspiration and has been reported to more than double the risk of stillbirth and neonatal death.25,28

Maternal risks associated with maternal obesity

Antenatal risks

Obesity in the pregnant woman is associated with an increased risk of gestational diabetes and hypertensive disorders of pregnancy including pre-eclampsia. The risk escalates with the degree of obesity; higher odds ratios reported in morbidly obese mothers.24,27,29 Obesity causes practical difficulties that obstetricians and midwives are only too aware of, including inaccuracies of abdominal palpation of the pregnant abdomen to assess the growth, lie, or presentation of the fetus, potential errors in sonographic prenatal diagnosis and errors in blood pressure measurement.

When maternal BMI is above the 90th centile, a significant reduction in the prenatal ultrasound scan diagnosis of fetal heart, spine and abdominal wall abnormalities has been shown.30 Among nonobese women, advancing gestation is the best predictor of visualisation of fetal anomalies, while among obese women, there is no improvement with advancing gestation or duration of the examination, with BMI being the best predictor. As this obese population is at higher risk of fetal abnormalities, this is of serious concern.

Large cuffs are required for blood pressure measurement, raising technical difficulties when the BMI is more than 40. Despite their practical appeal, none of the portable blood pressure monitoring devices is accurate for use by obese pregnant women with large upper arm circumferences of more than 35 cm.31

A maternal BMI in the obese range is predictive of an increased occurrence of cholelithiasis during pregnancy, accompanied by an increased risk of cholecystectomy in the first-year postpartum.32,33 Obesity is an acknowledged risk factor for thromboembolism in pregnancy, which remains the leading cause of maternal death in the UK.10

Intrapartum risks

A population-based observational study in the UK in 2001 involving 287 213 pregnancies in London and a subsequent study in 2005 on 60 167 deliveries in Wales showed that maternal obesity increased the risks of induction of labour, caesarean section, failed instrumental delivery and postpartum haemorrhage.11,34 Studies from the USA and other parts of the world reflect similar links between maternal obesity and intrapartum complications.4

Obesity is associated with a reduced likelihood of vaginal birth after caesarean sections (VBAC) and a reduced likelihood of vaginal delivery if weight gain in pregnancy exceeded 18 kg (40 lb).35 The success rate for VBAC in morbidly obese woman was found to be less than 15% in a study from Mississippi, whereas the chances of a successful VBAC in obese women was 68.2% in comparison with 79.9% for normal-weight women in a New York study.35,36 Obesity was found to be an independent adverse risk factor for VBAC in an obese and morbidly obese cohort of women in a Chicago study involving 725 women.37

A review of complications of Danish, singleton cephalic-term pregnancies in obese women reported a lower elective caesarean section rate but concluded that in addition to an increased emergency caesarean section rate, there was an increased risk of instrumental delivery and perineal tears.25 Although the overall risk of perineal tears was increased, no significant increase in third- or fourth-degree perineal tears was noted in a subsequent UK study.34 These risks and complications led to an increased length of hospital stay and cumulative health service cost per patient.

Anaesthetic risks

The high incidence of obstetric comorbidities leads to a greater likelihood of anaesthetic intervention. Failed regional blocks and difficult or failed intubation are more common, especially in the morbidly obese group. Additional risk factors such as maternal pre-eclampsia and diabetes further increase anaesthetic risk.38–44 Obesity can significantly impair respiratory function in women receiving spinal anaesthesia as height of block is positively correlated to BMI, contributing to slower recovery. Hypoxic complications due to an upward shift of diaphragm have also been reported.39–43

Postpartum risks

Obesity is independently associated with postcaesarean infectious morbidity. This association is retained with elective caesarean sections and the use of prophylactic antibiotics.45 Infections requiring open-wound debridement are significantly more common in morbidly obese women undergoing vertical abdominal incisions, unrelated to type of suture or use of subcutaneous drains.36,46,47 However, a significant reduction of wound dehiscence after caesarean section was demonstrated in a meta-analysis comparing subcutaneous layer closure with nonclosure.48

Obese women are at increased risk of postpartum urinary tract infections and postpartum anaemia.34,49 They experience an increased risk of lactation failure and delay in establishing lactation postdelivery, resulting in higher formula feeding rates that increase the risk of childhood obesity.50–52

Long-term risks

Obese women who gain weight during pregnancy are at risk of retaining this additional weight in the long term. This can lead to poor quality health, including increased mortality and morbidity from coronary heart disease, diabetes, hypertension, stroke and a number of cancers.53,54 Undeniably, all the above factors can be associated with negative self and body image, potentially predisposing to poor mental health.55

How can we make pregnancy safer for obese women: acknowledge as ‘high-risk pregnancy’

A (nonsystematic) review of the literature using medline keywords ‘obesity in pregnancy’ and ‘nutrition in pregnancy’ revealed an abundance of observational data, including one detailed systematic review published in 2000 on weight gain in pregnancy.4 No interventional studies in pregnant obese women was identified. One study evaluated the efficacy of an intervention to prevent excessive gestational weight gain in overweight, but not obese, women.56 The Diabetic and Obesity research network based in Manchester, UK, has a study in progress involving obese women with diabetes.57 While the American College of Obstetricians and Gynaecologists has recently issued a guidance statement on the subject in August 2005,58 the Royal College of Obstetricians and Gynaecologists is yet to issue any peer-reviewed guidelines on the management of maternal obesity in pregnancy.

This study highlights that pregnancy for an obese mother could be considered a high-risk pregnancy associated with fetal and maternal risks throughout the antenatal, intrapartum and postpartum period and extending well beyond the birth of the baby. It is probably time that obstetricians acknowledge this fact and consider an integrated, multidisciplinary approach to care plan involving primary care physicians, obstetricians, anaesthetists, midwives, dieticians, exercise advisors and other specialists depending on maternal complications.

Guidelines for the management of the obese (pre) pregnant woman

Pre-pregnancy care

Obese women may benefit from pre-pregnancy counselling regarding specific problems associated with obesity in pregnancy and advised to aim for a moderate weight loss prior to conception. Few data exist on effects on maternal and fetal outcomes following pre-pregnancy weight loss, although epidemiological studies have indicated that even a modest reduction of 10 pounds can reduce the risk of gestational diabetes among obese women.58,59

The increased risk of neural tube defects in obese mothers strengthens the need for preconception folate supplementation. It is questionable whether these women would benefit from higher dose of folates, as recommended in mothers with diabetes. This can only be answered by randomised controlled trials. A strategy is key to the successful delivery of an integrated multidisciplinary care plan.

Planning and developing services within individual units

Strong arguments exist for planning and developing services for obese women within maternity departments either within an existing ‘medical antenatal clinic’ for mothers with other medical disorders of pregnancy or, given the high prevalence of obesity, as a separate ‘maternal obesity clinic’. In-line with planning service provision based on local population needs,60 the starting point would be to assess the local prevalence rate of obesity in pregnancy.

In their latest report, the CEMACH committee has recommended that obese women with a BMI greater than 35 may be unsuitable for entirely midwife-led care and it is recommended that obese women give birth in a consultant unit with appropriate emergency facilities.10

Care in pregnancy

Recent guidance,61 precluding the routine weighing of healthy pregnant women, may not, in view of the increased risks described above, be applicable to overweight and obese women. The pre-pregnancy BMI and booking BMI should ideally be recorded at the first visit in the first trimester, followed by regular monitoring of gestational weight gain throughout the pregnancy.

The woman should be counselled regarding the implications of obesity on the course of pregnancy and its outcome, the proposed care plan to address these implications and this discussion needs to be clearly documented. She is likely to benefit from consistent support and advice regarding preventing excess weight gain, supported by written information. Regular visits to the dietician incorporating dietary and healthy lifestyle advice including exercise may also prove beneficial.62

A systematic review of available observational data published in 2000 revealed that pregnancy weight gain within the IOMs recommended range is associated with the best outcome for both mothers and infants.4 The IOM recommendations on weight gain could be explained to obese women at booking and reiterated at follow-up visits.

The increased risk of fetal congenital abnormalities, compounded by the difficulties of poor sonographic visualisation, suggests that anomaly scans in these women should ideally be performed by an ultrasonographer with an appropriate level of expertise. The increased risk of gestational diabetes indicates the need for screening for this condition.

As estimation of fetal size, lie, presenting part and clinical growth assessment based on abdominal palpation can be unreliable in obese women, serial growth scans could be considered especially in the morbidly obese group. However, scanning these women for fetal weight and wellbeing could be increasingly difficult and overinvestigation may potentially increase the operative intervention in this group who are at risk of surgical difficulty and thromboembolism. Care needs to be taken with regular blood pressure measurement using a large cuff and validated apparatus.

Antenatal anaesthetic review is recommended for those at the higher end of the BMI range (for example BMI > 35). Technical difficulties with regional anaesthesia can be overcome with needle sets specifically designed for morbidly obese women. A high initial failure rate with regional anaesthesia necessitates early catheter placement, critical block assessment and catheter replacement when indicated and provision for alternative airway management, for example fibre optic airway introduction. It should be remembered that the height of the regional block is positively correlated to the maternal BMI.

Whether induction of labour is necessary will depend, as in nonobese women, on coexisting complications, for example severe pre-eclampsia or poorly controlled diabetes. While aiming for a normal vaginal birth, there must be a heightened awareness of the increased likelihood of failed induction of labour, slow progress, fetal distress and the risk of failed instrumental delivery leading to emergency caesarean section. Vigilance for the occurrence of fetal macrosomia or infectious morbidity is also required. The increased incidence of postpartum haemorrhage calls for timely administration of uterotonics. The higher risk of postnatal anaemia requires a low threshold to treat with iron supplements. Continued support and encouragement to persevere with breastfeeding may avoid the lactation failure that is more common in overweight and obese women. Expert care is required for those obese women who have undergone bariatric surgery as they are prone to develop complications such as gastrointestinal bleeding, calcium deficiency and anaemia related to deficiency of iron, vitamin B12 and folate. Infants born to these women may develop intrauterine growth restriction. Pregnancy is best avoided for 12–18 months after surgery during the rapid weight loss phase. Vitamin supplementation are required if nutritional deficiencies occur.58

A recent prospective Australian study evaluating the results of laparoscopic adjustable gastric banding in severely obese women concluded that neonatal outcomes including stillbirth and preterm deliveries and maternal outcomes of pregnancy-induced hypertension and gestational diabetes were consistent with community values. This study also commented that the adjustability of banding allowed adaptations to altered requirements of pregnancy.63 These women should be monitored by their general surgeons during pregnancy because adjustment of the band may be necessary.58

Ideally, support should continue in the community setting after discharge from maternity care, encouraging women to adhere to diet, exercise and healthy lifestyle measures in the long term. Public health campaigns designed for women in the reproductive phase of life aimed at understanding the health effects of obesity may well support these changes.

Practical problems related to handling and moving patients and fetal monitoring in maternal obesity

The Obstetric Anaesthetic Association in conjunction with the Association of Anaesthetists of Great Britain has recommended that the operating table in all maternity theatres must be able to support a weight of at least 160 kg, with alternative provision for women who exceed this.44 Maternity staff training in moving and handling that includes specific measures for obese pregnant women could potentially improve ergonomic resourcefulness of staff. Morbidly obese women and those with impaired mobility require special hoists.

Fetal monitoring by intermittent auscultation or continuous cardiotocography (CTG) using external abdominal transducers can be technically difficult as the depth of maternal adipose tissue can interfere with the Doppler signal. This leads to poor quality CTGs that are difficult to interpret. Difficult to interpret CTGs, in association with a higher incidence of fetal distress and meconium aspiration in these women, could lead to poor neonatal outcomes and subsequent litigation. The use of fetal scalp electrodes to ensure an acceptable standard of fetal monitoring discussed with the woman in the antenatal period enables appropriate informed choice of the fetal monitoring method.

The need for research on obesity in pregnancy

Key national drivers and weaknesses/gaps in policy framework

The government pledged a commitment to the nation, through the NHS plan, to provide health care of a consistently high standard, tailored to the individual needs of patients, with emphasis on preventive care, to help tackle health inequalities.64 National standards local action65 sets out standards to improve the health of the population with key target areas around, reducing health inequalities, improving life expectancy and targeting the identification and management of obesity.

The National Service Framework for Diabetes asserts that women with gestational diabetes can reduce their risk of developing type II diabetes later in life by increasing their physical activity levels, eating a balanced diet and avoiding excessive weight gain.66 The National Service Framework for children, young people and maternity services 67 promotes flexible individualised services to the pregnant woman and her baby, with emphasis on the needs of vulnerable and disadvantaged women. By providing health care based on good clinical and psychological outcomes, with emphasis on preparation for parenthood, women and their partners must be supported and encouraged to have as normal a pregnancy and birth as possible.

It is disappointing that despite the high prevalence of obesity with implications for maternal and fetal health, these policy initiatives do not otherwise robustly address this problem. This may partly be due to the paucity of an evidence base to inform any recommendations, indicating an urgent need for good quality research.

Areas for future research

Several studies in the nonpregnant obese population confirm the health benefits of moderate weight reduction (10% of body weight) on glycaemic control, hypertension, cholesterol levels, risk of stroke, coronary heart disease and death.68 While data exist from case reports and case series on the use of antiobesity drugs and bariatric surgery pre-pregnancy,63,69,70 the review of the literature failed to find a single interventional trial on the effect of weight reduction in obese women pre-pregnancy and/or during pregnancy.

Whether a moderate weight reduction before pregnancy of 10% of body weight and/or strict weight gain restriction as per IOM recommendations will reduce the adverse fetomaternal outcomes described above requires such an interventional trial to determine. A reduction in weight may result in fewer interventions, e.g. caesarean sections. As a 1% reduction of the caesarean section rate can save the NHS 5 million pounds a year, this could result in economic as well as health benefits.71

High-quality health care requires an evidence-based culture built on well-designed clinical research. Service providers must actively engage in well-designed, ethical clinical research that aim to improve care for pregnant and recently pregnant women and their babies.67

Obesity is a major global problem that will only get worse if we do not act without further delay and with a sense of urgency.1