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Maternal Obesity, Mode of Delivery, and Neonatal Outcome

Blomberg, Marie MD, PhD

doi: 10.1097/AOG.0b013e318295657f
Original Research

OBJECTIVE: To evaluate whether adverse neonatal outcome, defined as birth injuries or severe illnesses in the newborn, was associated with maternal body mass index (BMI) in singleton pregnancies overall and depending on mode of delivery.

METHODS: This was a cohort study including 1,024,471 women. Data were collected from the Swedish Medical Birth Registry. Women were categorized into six classes of BMI. Obese women were compared with normal weight women regarding adverse neonatal outcome after suitable adjustments. Four modes of delivery were evaluated: vaginal delivery; instrumental vaginal delivery; elective cesarean delivery; and emergency cesarean delivery.

RESULTS: Compared with neonates born to women of normal weight, neonates born to women with BMIs of 40 or more (morbidly obese) were at increased risk of birth injury to the peripheral nervous system (odds ratio [OR] 3.80, 95% confidence interval [CI] 2.83–5.12; 0.2% compared with 0.6%), birth injury to the skeleton (OR 2.59, 95% CI 2.10–3.21; 0.5% compared with 1.1%), respiratory distress syndrome (OR 2.08, 95% CI 1.88–2.30; 2.9 compared with 5.8%), bacterial sepsis (OR 2.90, 95% CI 2.43–3.46; 0.6% compared with 1.7%), convulsions (OR 3.43, 95% CI 2.63–4.47; 0.2% compared with 0.8%), and hypoglycemia (OR 3.48, 95% CI 3.20–3.78; 2.4% compared with 7.9%). For morbidly obese women, elective cesarean delivery and vaginal delivery were associated with twice the increased risk of adverse neonatal outcomes when compared with women of normal weight.

CONCLUSION: Neonates born to morbidly obese women are at markedly increased risk of adverse neonatal outcome regardless of mode of delivery. Obstetricians should not disregard the neonatal problems associated with elective cesarean delivery for morbidly obese women.


Morbidly obese women are at increased risk of adverse neonatal outcomes regardless of mode of delivery.

Department of Clinical and Experimental Medicine, Linköping University, and the Department of Obstetrics and Gynecology, County Council of Östergötland, Linköping, Sweden.

Corresponding author: Dr. Marie Blomberg, MD, PhD, Department of Obstetrics and Gynecology, County Council of Östergötland, SE-581 85 Linköping, Sweden; e-mail:

Supported by the Östergötland County Council.

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

The author thanks the National Board of Health and Social Welfare, Stockholm, for access to the health registers.

Maternal obesity and especially maternal morbid obesity are associated with unfavorable neonatal outcome. Neonatal outcomes commonly related to maternal obesity are the size of the newborn, low Apgar score at 5 minutes, and neonatal death.1–8 To some extent, the association between maternal obesity and neonatal jaundice, neonatal hypoglycemia, and respiratory distress syndrome (RDS) have been evaluated, but results differ and some data sets are limited in size.8–11 Another approach to exploring short-term neonatal outcome among obese women is estimating the number of newborns admitted to neonatal intensive care units (NICU).8,11–13 Data concerning NICU admission are difficult to interpret and may reflect local traditions. Knowledge about the association between maternal obesity and adverse neonatal outcomes such as convulsions of the newborn, birth injuries to the central and peripheral nervous systems, and bacterial sepsis of the newborn is sparse.

Maternal obesity has reached epidemic proportions. Obstetricians in most countries care for pregnant obese and morbidly obese women daily.9,14,15 The prevalence of morbid obesity (body mass index [BMI, calculated as weight (kg)/[height (m)]2] 40 or more) among U.S. women aged 20–39 years in 2007–2008 was 7.6%. The corresponding figures concerning morbid obesity in early pregnancy were 1.4% in the United Kingdom, 2.0% in Australia, and 1% in Sweden.

The objectives of the present study were to estimate, in a large data set from the Swedish Medical Birth Registry, the association between maternal morbid obesity and adverse neonatal outcome, defined as birth injuries or severe illnesses in the newborn, and to determine whether maternal morbid obesity affects the risk of adverse neonatal outcome according to mode of delivery.

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The study population consisted of 1,024,471 women with singleton live births in Sweden from January 1, 1998 through December 31, 2008. They were identified from the Swedish Medical Birth Registry. Medical and other data for almost all (99%) deliveries in Sweden are listed in the register. It is based on copies of the standardized medical record forms completed at the antenatal health care centers at the start of prenatal care, usually during gestational week 10–12, records from the delivery units, and records from the pediatric examination of the newborn. Ninety percent of women present to the antenatal health care center during the first trimester of pregnancy. The system is identical throughout the country. A description and validation of the register content are available.16

Midwives measured maternal weight and height (if not known to the woman) and recorded values in a standardized form at the woman's first visit to the antenatal health care center. The BMI then was calculated from maternal weight and height data. Women were grouped into six categories of BMI according to the World Health Organization classification as follows: underweight (less than 18.5), normal weight (18.5–24.9), overweight (25–29.9), obese class I (30–34.9), obese class II (35–39.9), and obese class III (40 or more).17 Obesity class III is equivalent to morbid obesity in this study.

The following primary neonatal outcomes were studied:

  • Intracranial laceration, subdural hemorrhage, cerebral hemorrhage, intraventricular hemorrhage, and subarachnoid hemorrhage attributable to birth injury and other birth injuries to the central nervous system such as cerebral edema, specified brain damage, and injuries to the cranial nerves and spinal cord (International Classification of Diseases, 10th Edition [ICD-10] code P10-11).
  • Birth injury to the skeleton, including fracture of the skull, femur, other long bones, clavicle, or others (ICD-10 code P13).
  • Birth injury to the peripheral nervous system, including Erb paralysis, Klumpke paralysis, phrenic nerve paralysis, and other brachial plexus birth injuries (ICD-10 code P14).
  • Birth asphyxia (ICD-10 code P21), including severe birth asphyxia defined as pulse less than 100 per minute at birth and declining or steady, respiration absent or gasping, color poor, tone absent, and moderate birth asphyxia defined as normal respiration not established within 1 minute, but heart rate 100 bpm or more, some muscle tone present, and some response to stimulation.
  • Respiratory problems such as neonatal aspiration syndromes and pulmonary hemorrhage (ICD-10 code P22-28).
  • Bacterial sepsis of the newborn (ICD-10 code P36).
  • Convulsions of the newborn and other disturbances of cerebral status of the newborn (ICD-10 code P90-91).
  • Feeding problems of the newborn (ICD-10 code P92).
  • Hypoglycemia (ICD-10 code P70), a transitory disorder of neonate carbohydrate metabolism that includes the newborn being affected by maternal gestational diabetes or maternal preexisting diabetes mellitus, neonatal diabetes mellitus, and iatrogenic neonatal hypoglycemia.

The outcomes studied were registered in the Swedish Medical Birth Registry using the ICD-10. The ICD codes for each outcome have been presented.

The secondary outcome was to evaluate the risk of adverse neonatal outcomes according to mode of delivery over the six BMI strata. Four different modes of delivery were assessed, noninstrumental vaginal delivery, instrumental vaginal delivery, emergency cesarean delivery, and elective cesarean delivery. Noninstrumental vaginal delivery was defined as no mark in the register for cesarean delivery, vacuum extraction, or forceps. Instrumental delivery was defined as a mark in the registry for either vacuum extraction or forceps but not cesarean delivery. Any cesarean delivery was defined as a mark in the registry for cesarean delivery. The requirements for emergency and elective cesarean delivery, respectively, were both a mark in the registry for cesarean delivery and a further classification into elective cesarean delivery or emergency cesarean delivery. The classification into elective or emergency cesarean delivery was introduced in 1999 and is not complete in the register. The rate of unspecified cesarean deliveries has decreased from 39% in 2000 to 7.2% in 2008. Deliveries starting instrumentally and ending with cesarean delivery were classified as cesarean delivery.

Maternal age (seven 5-year classes), parity (one–four or more), smoking during early pregnancy (unknown, no smoking, fewer than 10 cigarettes/d, 10 or more cigarettes/d), and year of birth were thought to be potential confounding factors and were included as covariates in the adjusted analyses. The register information for these variables was obtained from the antenatal care center records.

Adjusted odds ratios (ORs) were determined using the Mantel-Haenszel test.18 Estimates of 95% confidence intervals (CIs) were made with a test-based method19 based on the Mantel-Haenszel χ2. The Regional Ethical Review Board in Linköping approved the study.

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Maternal age, parity, and smoking as potential risk factors for adverse neonatal outcome are shown in Table 1. Women who gave birth to newborns with adverse neonatal outcomes were slightly older, more often primiparous, and smokers compared with women giving birth to healthy newborns.

Table 1

Table 1

A total of 884,181 (86.3%) women had available data in the register about weight and height that enabled calculation of maternal BMI. The overall prevalence of obesity (BMI more than 30) was 10.8% and the distributions in the three obesity classes were 7.7% for class I, 2.3% for class II, and 0.8% for class III.

The risks for adverse neonatal outcome in six different maternal BMI classes are shown in Table 2. Normal-weight women (BMI 18.5–24.9) were used as a reference group. The risk of birth injuries to the skeleton and peripheral nervous system increased with increasing BMI, reaching a threefold to fourfold increase among morbidly obese women. Birth injuries to the central nervous system were rare; therefore, the risk estimates were difficult to interpret. The only statistically significant association was found in obesity class I. Birth asphyxia and convulsions of the newborn were strongly associated with maternal obesity. Bacterial sepsis of neonates born to obese mothers was significantly more common, as were feeding problems of the newborns and hypoglycemia. The risk of RDS increased with increasing maternal BMI. Newborns of underweight women (BMI less than 18.5) are at decreased risk of birth injuries to the skeleton, birth injuries to the peripheral nervous system, birth asphyxia, and convulsions.

Table 2-a

Table 2-a

Table 2-b

Table 2-b

In Figure 1, the risk associated with maternal obesity for any adverse neonatal outcome included in the present study is shown for each mode of delivery. The risk associated with maternal BMI increased with increasing maternal BMI regardless of mode of delivery.

Fig. 1

Fig. 1

The risk estimates (ORs) were 2.57 (95% CI 2.34–2.82) among morbidly obese women with normal vaginal deliveries and 2.08 (95% CI 1.68–2.58) for morbidly obese women with elective cesarean deliveries. The mode of delivery with the lowest risk of adverse neonatal outcomes among morbidly obese women was emergency cesarean delivery. Neonatal risks in the morbidly obese group associated with instrumental delivery were not higher than those for morbidly obese women with noninstrumental vaginal deliveries.

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This large, population-based, cohort study based on Swedish Medical Birth Registry data showed that neonates born to morbidly obese women have a doubled risk of birth injuries to the skeleton and RDS, a threefold increased risk of bacterial sepsis, convulsions, birth asphyxia, and feeding problems, and a fourfold increased risk of birth injuries to the peripheral nervous system and hypoglycemia. The risks of any adverse neonatal outcome seemed to increase with increasing maternal BMI, regardless of mode of delivery.

There are few studies evaluating neonatal outcome in relation to maternal morbid obesity. Adverse neonatal outcomes are rare and large data sets are required A doubled risk of neonatal death among obese women has been previously described2–4,20 but because of uncertain data in the Medical Birth Registry attributable to underreporting, neonatal death was not included in the present study. In a cohort study from Australia, an increased risk in obesity class III was found for neonatal RDS (adjusted OR 1.56, 95% CI 1.24–1.97) and for need of mechanical ventilation (adjusted OR 1.82, 95% CI 1.49–2.22).9,13 That is in accordance with the present results. The association between neonatal bacterial sepsis, which could be a life-threatening complication that needs to be treated at the NICU, and maternal morbid obesity was a strong risk factor in the present study. One study including 70 cases of generalized newborn infections found risks of 1.6% among women with BMI 40–44.99 and 2.4% among women with BMI 45 or more. The corresponding prevalence rate in the normal weight group (BMI 18.5–24.99) was 0.8%.10 The risk rate (adjusted OR) for hypoglycemia in the morbidly obese group was found to be 2.14 (95% CI 1.45–3.15), which was half of the risk found in the present study despite similar adjustments.9,10 In the present study, the birth injuries were divided into three groups, central nervous system, peripheral nervous system, and skeleton; all three were strongly associated with maternal morbid obesity. A study comparing obese women with nonobese women concluded a 50% increased risk of neonatal trauma (superficial skin cuts, grazes, and bruises, and also fractures, muscle hematomas, dislocations, cephalohematomas, and nerve palsies).11 Feeding problems in the newborn period could be attributable to newborn cerebral deprivation. One study found an increased risk of tube feeding among newborns born to obese mothers.11 Far more studies have evaluated the risk of admission to NICU over the BMI strata and found an association with maternal obesity.11–13 Most of them are regional studies in which one could believe that the clinical guidelines for admission to NICU are quite similar, making comparison over time possible. The present cohort study covers a whole country and, unfortunately, the indications for admission to NICU varies widely, mainly related to local resources and patient loads instead of being related to clear clinical indications.

Opinions concerning optimal delivery in morbidly obese women are many and are not always evidence-based. Knowledge is sparse and there is a lack of comparative studies of maternal and neonatal outcomes between the different modes of delivery. This study was a brief effort to, in some way, illuminate this issue. The risk of adverse neonatal outcome increased with increasing BMI regardless of mode of delivery. The risk was highest for vaginal delivery, but it did not differ significantly from risk estimates for elective cesarean delivery, which may appear surprising. The lowest risk associated with maternal obesity was found in the emergency cesarean delivery group. These findings need to be interpreted with caution because of possible limitations in the study design, but one clinical explanation could be the lower incidence of RDS in this group compared with the elective cesarean delivery group. However, RDS was the outcome with highest prevalence included in this study.

The advantage of population-based register studies is the large number of individuals available for evaluation, which makes it possible to divide the study population into subgroups with sufficient numbers in each stratum and gives high statistical power. A sufficient number of study participants made it possible to evaluate the three subgroups of obesity suggested by World Health Organization obesity classes I–III. Problems concerning studies in this field, especially when estimating risk rates for rare outcomes, are the definition and the cut-off values for obesity, making it difficult to exactly compare risk estimates. The control groups used in different studies also vary and could include underweight women or nonobese women, making interpretations of data difficult. An advantage is access to information is available in the register regarding putative confounders. The drawback is that because of the large size of the study and the number of health care units involved, the criteria for diagnosis (ICD codes) to define neonatal outcomes may not be uniform across the study population, but the variation probably is not related to maternal BMI.

To demonstrate causality between the different neonatal outcomes included in the analysis and maternal morbid obesity, a great number of putative intermediaries would have been considered such as fetal size, time in labor, and oxytocin infusion, but that was not the purpose of this study. Instead, the effort was to evaluate adverse neonatal outcomes in the obese and morbidly obese groups of women overall. The only stratifications made were for maternal age, parity, year of birth, and smoking during early pregnancy. This approach may be a benefit for clinicians interpreting data when dealing with obese mothers.

In conclusion, morbidly obese women are at increased risk of a number of adverse neonatal outcomes and there is little difference between these increased risks and the different modes of delivery. Results from this study indicate no protective effect of elective cesarean delivery for adverse neonatal outcomes among morbidly obese women.

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