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Keywords:

  • Birthweight;
  • body mass index;
  • fetal hyperinsulinism;
  • hyperglycaemia;
  • large for gestational age

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References
  8. Supporting Information

Please cite this paper as: HAPO Study Cooperative Research Group. Hyperglycaemia and Adverse Pregnancy Outcome (HAPO) Study: associations with maternal body mass index. BJOG 2010;117:575–584.

Objective  To determine whether higher maternal body mass index (BMI), independent of maternal glycaemia, is associated with adverse pregnancy outcomes.

Design  Observational cohort study.

Setting  Fifteen centres in nine countries.

Population  Eligible pregnant women.

Methods  A 75-g 2-hour oral glucose tolerance test (OGTT) was performed between 24 and 32 weeks of gestation in all participants. Maternal BMI was calculated from height and weight measured at the OGTT. Fetal adiposity was assessed using skinfold measurements and percentage of body fat was calculated. Associations between maternal BMI and pregnancy outcomes were assessed using multiple logistic regression analyses, with adjustment for potential confounders.

Main outcome measures  Predefined primary outcomes were birthweight >90th percentile, primary caesarean section, clinical neonatal hypoglycaemia and cord serum C-peptide >90th percentile. Secondary outcomes included pre-eclampsia, preterm delivery (before 37 weeks) and percentage of body fat >90th percentile.

Results  Among 23 316 blinded participants, with control for maternal glycaemia and other potential confounders, higher maternal BMI was associated (odds ratio [95% confidence interval] for highest {≥42.0 kg/m2} versus lowest {<22.6 kg/m2} BMI categories) with increased frequency of birthweight >90th percentile (3.52 [2.48–5.00]) and percentage of body fat >90th percentile (3.28 [2.28–4.71]), caesarean section (2.23 [1.66–2.99]), cord C-peptide >90th percentile (2.33 [1.58–3.43]) and pre-eclampsia (14.14 [9.44–21.17]). Preterm delivery was less frequent with higher BMI (0.48 [0.31–0.74]). Associations with fetal size tended to plateau in the highest maternal BMI categories.

Conclusion  Higher maternal BMI, independent of maternal glycaemia, is strongly associated with increased frequency of pregnancy complications, in particular those related to excess fetal growth and adiposity and to pre-eclampsia.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References
  8. Supporting Information

Maternal overweight and obesity are associated with increased risks of adverse pregnancy outcomes, in particular maternal complications such as hypertensive disorders of pregnancy,1 gestational diabetes2–4 and maternal mortality.5 Fetal or neonatal complications including stillbirth,6 birth defects,7 macrosomia3,8 and shoulder dystocia9 also occur more frequently in babies born to obese women but findings are more variable for other putative complications such as preterm delivery2,3,10 and early neonatal death.9,10

Obesity is also associated with an increased risk of diabetes both during and outside pregnancy. Previous reports do not allow a clear separation of the relative importance of obesity per se and hyperglycaemia as contributors to increased risks of adverse pregnancy complications because of unavoidable methodological weaknesses. In some reports routine screening for hyperglycaemia was not undertaken,2,3 whereas in others the approach to screening was not outlined.2 In studies which explicitly report glucose screening of the entire cohort, detailed analyses of the prevalence of gestational diabetes in overweight and obese women were not reported.11,12 Furthermore, in none of the previous studies were glucose results blinded from women or their caregivers. This may have influenced both clinical decision-making and ascertainment of pregnancy outcomes.

The Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study was designed primarily to assess the effects of mild hyperglycaemia on pregnancy outcomes.13,14 Glucose results were blinded from women and caregivers, except where results fell outside predetermined ranges.14

The HAPO study previously demonstrated relationships between fasting and postload glucose concentrations measured during a third-trimester 75-g oral glucose tolerance test (OGTT) and pregnancy outcomes, including fetal size, adiposity and hyperinsulinism. These were only modestly attenuated after correction for other factors, including maternal body mass index (BMI).14,15

For the current analysis, our hypothesis was that higher maternal BMI is associated with adverse pregnancy outcomes and neonatal adiposity independent of other factors, including maternal glycaemia.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References
  8. Supporting Information

The HAPO study was an international, multicentre epidemiological study conducted at 15 centres in nine countries. Methods have been reported elsewhere.13,14 The study was approved by the local institutional review board at each centre. All participants gave written informed consent and the study was overseen by an external Data Monitoring Committee.

Participants

All pregnant women at each centre were eligible to participate unless they had one or more exclusion criteria: age <18 years, planning to deliver at another hospital, date of last menstrual period not certain and no ultrasound estimation from 6 to 24 weeks of gestation available, unable to complete the OGTT by 32 weeks of gestation, multiple pregnancy, conception using gonadotrophin ovulation induction or by in vitro fertilisation, glucose testing before recruitment or a diagnosis of diabetes during this pregnancy, diabetes antedating pregnancy requiring treatment with medication, participation in another study which may interfere with HAPO, known to be human immunodeficiency virus-positive or to have hepatitis B or C virus infection, before participation in HAPO, or inability to converse in the languages used in field centre forms without the aid of an interpreter.13 Gestational age and expected date of delivery were determined from the date of the last menstrual period, if the participant was certain of her dates. If uncertain, expected date of delivery was determined from an ultrasound performed between 6 and 24 weeks of gestation. Final expected date of delivery was also determined from ultrasound if: (1) gestational dating from last menstrual period differed from ultrasound dating by more than 5 days, when the ultrasound was performed between 6 and 13 weeks, or (2) if dating differed by more than 10 days when the ultrasound was performed between 14 and 24 weeks.

Oral glucose tolerance test

All participants underwent a standard 75-g OGTT between 24 and 32 weeks of gestation (as close to 28 weeks as possible). Blood pressure was measured at the OGTT visit using standardised procedures and calibrated equipment. Data concerning smoking and alcohol use, first-degree family history of diabetes and hypertension and demographic data were collected using standardised questionnaires. Race/ethnicity was self-identified by participants.

A sample for random plasma glucose was collected at 34–37 weeks of gestation as a safety measure to identify cases with hyperglycaemia above a predefined threshold.

Glucose analysis and unblinding

Aliquots of fasting and 2-hour OGTT and random plasma glucose samples were analysed at field centre laboratories. Values were unblinded if fasting plasma glucose exceeded 5.8 mmol/l, if 2-hour OGTT plasma glucose exceeded 11.1 mmol/l, if random plasma glucose was ≥8.9 mmol/l or if any plasma glucose value was <2.5 mmol/l. Otherwise, women, caregivers and HAPO Study staff (except laboratory personnel) remained blinded to glucose values. To avoid the confounding effects of centre-to-centre analytical variation, aliquots of all OGTT specimens were analysed at the HAPO Central Laboratory and those results are used here. Only women whose results remained blinded, with no additional glucose testing outside the HAPO protocol, are included in these analyses.

Maternal height and weight

The primary objective measurements of maternal height and weight, used to calculate BMI in this report, were obtained at the OGTT visit. Height was measured twice to the nearest 0.5 cm with a stadiometer or wall-mounted measuring tape with shoes removed and the participant’s head facing forward in the horizontal plane. If the results differed by more than 1.0 cm, the measurements were repeated. Weight was measured twice to the nearest 0.1 kg on a scale calibrated each day with a 10-kg weight. Outer garments and shoes were removed and a third weight measurement was taken if the results of the first two measurements differed by more than 0.5 kg. Recalled maternal pre-pregnancy weight was also recorded, but is not the primary focus of this report because of its inherent subjectivity and the absence of data for 1966 participants (8.4%). No centre provided specific interventions to participants based on weight or BMI.

Prenatal care and delivery

Prenatal care and timing of delivery were determined by standard field centre practice. No field centre arbitrarily delivered babies before full term or routinely performed caesarean delivery at a specified maternal or gestational age.

Neonatal care and anthropometrics

After delivery, infants received customary routine care. Neonatal anthropometrics were obtained within 72 hours of delivery and included weight, length and skinfold thickness at three sites (flank, subscapular, triceps). Measurement and quality control procedures have been described elsewhere.15 Medical records were abstracted to obtain data regarding maternal and newborn course.

Primary and secondary outcomes

The four prespecified primary outcomes were birthweight above the 90th percentile for gestational age, primary caesarean delivery, clinical neonatal hypoglycaemia and cord serum C-peptide above the 90th percentile (fetal hyperinsulinaemia). Secondary outcomes were preterm delivery (before 37 weeks of gestation), shoulder dystocia or birth injury, need for more intensive neonatal care, hyperbilirubinaemia and pre-eclampsia. The 90th percentiles for gestational age (30–44 weeks only) were determined using quantile regression analyses for each of eight newborn gender–ethnic groups (Caucasian or Other, Black people, Hispanic, Asian), with adjustment for gestational age, field centre and parity (0, 1, 2+). A newborn was considered to have a birthweight >90th percentile if the birthweight was greater than the estimated 90th percentile for the baby’s gender, gestational age, ethnicity, field centre and maternal parity. Otherwise, the newborn was considered to have a birthweight ≤90th percentile.

Neonatal adiposity

Additional measures of neonatal adiposity included percentage of body fat >90th percentile and sum of skinfolds >90th percentile for gestational age. Fat mass was calculated from birthweight, length and flank skinfold using an equation derived from measurements of total body electrical conductivity.16 The percentage of body fat was calculated as 100 × fat mass/birthweight.

Statistical analyses

Descriptive statistics include mean and standard deviation for continuous variables and numbers and percentages for categorical variables. For associations of maternal BMI with outcomes BMI was considered as both a categorical and continuous variable in multiple logistic regression analyses. For categorical analyses, maternal BMI was divided into six categories. Category limits were chosen to match the recommended pre-pregnancy BMI categories of <18.5 (underweight), 18.5–24.9 (normal range), 25.0–29.9 (overweight), 30.0–34.9 (obese class I), 35.0–39.9 (obese class II) and ≥40.0 (obese class III).17 The comparable category limits for BMI at the OGTT were obtained from a regression of OGTT BMI on pre-pregnancy BMI and gestational age at the OGTT, and yielded the following categories at 28 weeks of gestation: <22.6, 22.6–28.4, 28.5–32.9, 33.0–37.4, 37.5–41.9, ≥42.0.

To assess whether or not the log of the odds of each outcome was linearly related to BMI, we added squared terms for BMI for each outcome to assess whether there were significant quadratic associations (see below). For each outcome, two logistic models (I and II) were fit. Model I included adjustment for centre or for the variables used in estimating the 90th percentiles for birthweight, percentage of fat and sum of skinfolds for gestational age (gender, ethnicity, centre and parity). In addition, Model I included adjustment for multiple potential confounders, including age, height, smoking, alcohol use, family history of diabetes, gestational age at the OGTT, baby’s gender, parity (0, 1, 2+) (except primary caesarean delivery), and hospitalisation before delivery (except pre-eclampsia), and family history of hypertension and maternal urinary tract infection (pre-eclampsia only). Model II added fasting glucose and mean arterial pressure to Model I, because of the potential for these variables, with their known associations with BMI, to be in the causal pathway of the associations of BMI with outcomes. Squared terms for age, fasting glucose and mean arterial pressure were prescreened for possible inclusion in models that included only centre; that is without BMI or other covariates; these terms were included in Model I and/or II when statistically significant (see below).

Squared terms for glucose, age, BMI and mean arterial pressure were considered statistically significant for P ≤ 0.001 for all outcomes, except neonatal hypoglycaemia and shoulder dystocia/birth injury, where P ≤ 0.05 was used because of the smaller numbers of babies with these outcomes. Because the squared term for BMI was statistically significant for several of the outcomes indicating nonlinear associations, only the categorical results are reported here.

All analyses were conducted in sas version 9.1 (SAS Institute Inc., Cary, NC, USA) or Stata 10.0 (StataCorp, College Station, TX, USA). All reported P-values are two-sided and were not adjusted for multiple testing.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References
  8. Supporting Information

Table 1 shows the characteristics of mothers and babies in the HAPO study and the overall frequencies of some obstetric outcomes. (More detailed information is presented in a previous publication14) The mean age of participants was 29.2 years, and the mean maternal BMI at the OGTT was 27.7 kg/m2. For the 21 350 participants with a recalled pre-pregnancy weight, the corresponding mean pre-pregnant BMI was 23.9 kg/m2 and the correlation with the BMI at the OGTT visit was 0.918. The mean (SD) difference in BMI from pre-pregnancy to 28 weeks of gestation was +3.6 (2.0) kg/m2, varying from +3.9 (1.6) in those with pre-pregnancy BMI <18.5 kg/m2 to +1.7 (2.9) in those with pre-pregnancy BMI ≥35.0 kg/m2. The field-centre-adjusted correlations of BMI at the OGTT with fasting, 1-hour and 2-hour glucose were 0.312, 0.197 and 0.154, respectively.

Table 1.   Characteristics of hyperglycemia and adverse pregnancy outcome (HAPO) study participants and frequency of outcomes
Maternal characteristicsNMeanSD
Age (years)23 31629.25.8
Pre-pregnancy body mass index (kg/m2) (self-reported weight)21 32423.95.0
Body mass index (kg/m2)*23 31627.75.1
Mean arterial pressure (mmHg)*23 31680.98.3
Fasting plasma glucose (mmol/l)*,**23 3164.50.4
1-hour plasma glucose (mmol/l)*,**23, 3167.51.7
2-hour plasma glucose (mmol/l)*,**23 3166.21.3
Gestational age (weeks)*23 31627.81.8
 N% 
Prenatal smoking (any)15816.8 
Prenatal alcohol use (any)16126.9 
Family history of diabetes528222.7 
Parity (prior delivery ≥20 weeks)12 23352.5 
Prenatal urinary tract infection16557.1 
Hospitalisation prior to delivery327114.0 
Newborn characteristicsNMeanSD
  1. *Measured at the OGTT visit.

  2. **For glucose to mg/dl, multiply by 18.

  3. †Babies with gestational age 36–44 weeks at birth.

  4. ‡Hypertension present before 20 weeks which did not progress to pre-eclampsia was classified as chronic hypertension. After 20 weeks of gestation, hypertension disorders in pregnancy were categorised according to International Society for the Study of Hypertension (ISSHP) guidelines.31 Pre-eclampsia, systolic blood pressure (BP) ≥ 140 mmHg and/or diastolic BP ≥ 90 mmHg on two or more occasions a minimum of 6 hours apart and proteinuria of ≥1+ dipstick or ≥300 mg/24 hours. If the criteria for elevated BP but not proteinuria were met, this was classified as gestational hypertension.

Gestational age (weeks)23 31639.41.7
Birthweight (g)23 2673,292529
Length (cm)22 60149.72.4
Body fat (%)†19 32211.33.7
Sex (male)12 00351.5 
Obstetric outcomesN% 
Caesarean section delivery
 Primary373116.0 
 Repeat17927.7 
Hypertension‡
 Chronic hypertension5822.5 
 Gestational hypertension13705.9 
 Pre-eclampsia11164.8 

The effects of including maternal 1-hour and 2-hour glucose results, in addition to fasting glucose in Model II adjustments, were examined for all outcomes reported here. These further adjustments resulted in little change in any of the odds ratios. Hence, we present the results with adjustment for fasting glucose alone.

Maternal BMI and primary outcomes

The relationships between maternal BMI at the OGTT and the prespecified primary outcomes are presented in Table 2. The frequency of birthweight >90th percentile was greater with higher maternal BMI with some reduction in the incremental effects seen in the highest BMI categories. Even after full Model II adjustments, the odds ratios for the four highest BMI categories were all >3.0

Table 2.   Relationship between maternal body mass index (BMI) at the oral glucose tolerance test (OGTT) and primary outcomes
BMI (kg/m2)N#%Model I OR95% CIModel II OR95% CI
  1. N, total number in the BMI category; #, number in the BMI category with the outcome; %, proportion in the BMI with the outcome.

  2. BMI category limits at the OGTT were chosen to match the recommended pre-pregnancy categories of <18.5 (underweight), 18.5–24.9 (normal range), 25.0–29.9 (overweight), 30.0–34.9 (obese class I), 35.0–39.9 (obese class II), and ≥40.0 (obese class III).

  3. *Ninetieth percentiles for gestational age (30–44 weeks only) were determined using quantile regression analyses for each of eight newborn gender–ethnic groups (Caucasian or Other, Black people, Hispanic, Asian), with adjustment for gestational age, field centre and parity (0, 1, 2 +). A newborn was considered to have a birthweight >90th percentile if the birthweight was greater than the estimated 90th percentile for the baby’s gender, gestational age, ethnicity, field centre, and maternal parity. Otherwise, the newborn was considered to have a birthweight ≤ 90th percentile.

  4. **Model I: Adjusted for the variables used in estimating 90th percentiles, age, height and gestational age at the OGTT, smoking, alcohol use, hospitalisation before delivery, any family history of diabetes. Model II: Model I adjustment + fasting plasma glucose and mean arterial pressure.

  5. †Model I: Adjusted for field centre, age, gender, smoking, alcohol use, hospitalisation before delivery, any family history of diabetes, gestational age and maternal height at the OGTT. Model II: Model I adjustment + fasting plasma glucose and mean arterial pressure.

  6. ‡Clinical neonatal hypoglycaemia was defined as present if there was notation of neonatal hypoglycaemia in the medical record and there were symptoms and/or treatment with a glucose infusion or a local laboratory report of a glucose value 1.7 mmol/l in the first 24 hours and/or ≤2.5 mmol/l after the first 24 hours after birth.

  7. §Same Models as for primary caesarean delivery except parity was added.

  8. ¶90th percentile of the values for the total HAPO sample.

  9. ††Same Models as for primary caesarean delivery except that parity and cord glucose were added.

Birthweight >90th percentile*,**
<22.629741153.91.00 1.00 
22.6–28.411 9349818.22.241.84–2.742.171.78–2.65
28.5–32.9512764412.63.622.95–4.453.312.68–4.10
33.0–37.4206430814.94.433.54–5.553.893.07–4.93
37.5–41.973511215.24.523.43–5.963.802.84–5.08
≥42.03836115.94.553.25–6.363.522.48–5.00
Total23 21722219.6    
Primary caesarean section
<22.6280935612.71.00 1.00 
22.6–28.410 868181416.71.271.12–1.441.171.03–1.33
28.5–32.9443891620.61.781.54–2.041.481.28–1.71
33.0–37.4170740423.72.261.91–2.671.751.47–2.08
37.5–41.960915225.02.461.96–3.071.781.41–2.25
≥42.03018929.63.202.41–4.252.231.66–2.99
Total20 732373118.0    
Clinical neonatal hypoglycaemia‡,§
<22.62978752.51.00 1.00 
22.6–28.411 9382251.90.810.62–1.070.760.58–1.01
28.5–32.95123991.90.890.64–1.220.730.52–1.03
33.0–37.42068502.41.080.74–1.580.820.54–1.23
37.5–41.9737182.41.190.69–2.040.820.47–1.45
≥42.0383133.41.740.93–3.261.110.58–2.15
Total23 2274802.1    
Cord serum C-peptide >90th percentile¶,††
<22.626331234.71.00 1.00 
22.6–28.410 3177317.11.461.19–1.791.291.05–1.58
28.5–32.9435544810.32.181.75–2.711.661.32–2.08
33.0–37.4168323213.83.112.44–3.972.131.65–2.75
37.5–41.95998313.93.042.23–4.151.901.37–2.63
≥42.02985418.14.302.98–6.202.331.58–3.43
Total19 88516718.4    

The frequency of primary caesarean section was also greater with higher BMI with an odds ratio of approximately 2.5 or greater for the two highest BMI categories after Model I adjustment. With additional adjustment for fasting glucose and mean arterial pressure, these two odds ratios were somewhat attenuated.

The frequency of clinical neonatal hypoglycaemia showed some tendency to rise with higher maternal BMI, but none of the odds ratios were significantly different from 1.0 when fully adjusted in Model II.

Neonatal hyperinsulinaemia, measured by cord C-peptide >90th percentile, was also positively associated with maternal BMI with odds ratios over 3.0 in the highest three categories in Model I. These odds ratios were attenuated in Model II, but were still ≥1.9.

Maternal BMI and secondary outcomes

Table 3 shows the relationships between maternal BMI and the prespecified secondary outcomes. The frequency of preterm delivery was significantly lower with higher maternal BMI in Model II. Pre-eclampsia was present in only 11.7% of cases of preterm delivery. Shoulder dystocia and birth injury were much less common events (overall frequency 1.3%). Their frequency was significantly higher than in the referent group for maternal BMI categories spanning the range from 28.5 to 37.4, both in Model I and Model II, but was not significantly increased in the highest two BMI categories, in part because of the small number of events in these two categories.

Table 3.   Relationship between maternal body mass index (BMI) at the oral glucose tolerance test (OGTT) and secondary outcomes
BMI (kg/m2)*N#%Model I OR95% CIModel II OR95% CI
  1. N, total number in the BMI category; #, number in the BMI category with the outcome; %, proportion in the BMI with the outcome.

  2. BMI category limits at the OGTT were chosen to match the recommended pre-pregnancy categories of <18.5 (underweight), 18.5–24.9 (normal range), 25.0–29.9 (overweight), 30.0–34.9 (obese class I), 35.0–39.9 (obese class II), and ≥ 40.0 (obese class III).

  3. *Model I: Adjusted for field centre, age, gender, parity, smoking, alcohol use, hospitalisation before delivery, any family history of diabetes, gestational age and maternal height at the OGTT. Model II: Model I adjustment + fasting plasma glucose and mean arterial pressure.

  4. **Hyperbilirubinaemia was defined by treatment with phototherapy after birth, at least one laboratory report of a bilirubin level of 342 μmol/l or more, or readmission for hyperbilirubinaemia.

  5. †Intensive neonatal care was defined as admission to any type of unit for care more intensive than normal newborn care and lasting more than 24 hours or as death of the baby or transfer to another hospital. Data were excluded for admissions that were only for possible sepsis and sepsis was ruled out, observation, or feeding problems.

  6. ‡Model I deleted hospitalisation before delivery and added maternal urinary tract infection and family history of hypertension. Model II deleted mean arterial pressure.

Preterm delivery (<37 weeks)*
<22.629892518.41.00 1.00 
22.6–28.411 9867686.40.790.68–0.920.720.62–0.84
28.5–32.951433667.10.880.74–1.050.690.57–0.83
33.0–37.420751276.10.730.58–0.920.500.39–0.64
37.5–41.9739679.11.130.84–1.510.700.51–0.95
≥42.0384297.60.880.58–1.330.480.31–0.74
Total23 31616086.9    
Shoulder dystocia/birth injury*
<22.62978210.71.00 1.00 
22.6–28.411 9381451.21.510.95–2.421.520.95–2.43
28.5–32.95123871.71.951.19–3.211.961.18–3.28
33.0–37.42068401.92.091.20–3.622.091.17–3.73
37.5–41.9737111.51.510.71–3.211.500.69–3.28
≥42.038371.81.720.71–4.161.670.67–4.16
Total23 2273111.3    
Hyperbilirubinaemia*,**
<22.629782869.61.00 1.00 
22.6–28.411 93810338.71.090.94–1.261.060.91–1.23
28.5–32.951233727.31.211.02–1.441.090.91–1.31
33.0–37.420681507.31.301.04–1.621.100.88–1.39
37.5–41.9737618.31.621.20–2.201.320.96–1.80
≥42.0383287.31.490.98–2.261.150.75–1.77
Total23 22719308.3    
Intensive neonatal care*,†
<22.629782689.01.00 1.00 
22.6–28.411 9389247.71.090.94–1.271.070.91–1.25
28.5–32.951233737.31.241.04–1.491.130.93–1.36
33.0–37.420681758.51.431.16–1.781.220.97–1.53
37.5–41.97378111.01.981.49–2.611.611.20–2.15
≥42.0383348.91.511.02–2.231.150.77–1.73
Total23 22718558.0    
Pre-eclampsia
<22.62893582.01.00 1.00 
22.6–28.411 2893683.31.661.25–2.211.561.17–2.08
28.5–32.945742936.43.252.41–4.372.852.11–3.85
33.0–37.4173919711.36.014.39–8.225.013.64–6.89
37.5–41.959611920.011.137.86–15.768.926.25–12.73
≥42.02738129.718.7712.67–27.8114.149.44–21.17
Total21 36411165.2    

The associations of maternal BMI with neonatal hyperbilirubinaemia and intensive neonatal care were positive in Model I. Few categories showed significant associations for these outcomes after Model II adjustment.

The frequency of pre-eclampsia showed a strong linear relationship with increasing maternal BMI. Women in the highest BMI category had an odds ratio >18.0 for pre-eclampsia, which was somewhat attenuated by Model II adjustment but was still greater than 14.0.

Neonatal adiposity

Consistent with the primary outcome of birthweight >90th percentile, both percentage of body fat and sum of skinfolds >90th percentile were more frequent with higher maternal BMI (Table 4). This confirms that these babies were not only large, but that they also had increased fat deposits. As seen for birthweight >90th percentile, odds ratios are somewhat attenuated in the highest BMI categories with Model II adjustment.

Table 4.   Relationship between maternal body mass index (BMI) at the oral glucose tolerance test (OGTT) and neonatal adiposity
BMI (kg/m2)N#%Model I OR95% CIModel II OR95% CI
  1. N, total number in the BMI category; #, number in the BMI category with the outcome; %, proportion in the BMI with the outcome.

  2. BMI category limits at the OGTT were chosen to match the recommended pre-pregnancy categories of <18.5 (underweight), 18.5–24.9 (normal range), 25.0–29.9 (overweight), 30.0–34.9 (obese class I), 35.0–39.9 (obese class II), and ≥40.0 (obese class III).

  3. *Defined based on gender, ethnicity, field centre, gestational age (36–44 weeks only), parity using the same methods as for birthweight >90th percentile (see footnote to Table 2).

  4. Model I: Adjusted for the variables used in estimating 90th percentiles, age, height and gestational age at the OGTT, smoking, alcohol use, hospitalisation before delivery, any family history of diabetes. Model II: Model I adjustment + fasting plasma glucose and mean arterial pressure.

Per cent body fat >90th percentile*
<22.625131134.51.00 1.00 
22.6–28.499628178.21.911.56–2.341.811.48–2.23
28.5–32.9423054412.93.152.55–3.882.772.22–3.44
33.0–37.4170125815.23.823.03–4.823.192.50–4.08
37.5–41.960410417.24.453.34–5.913.512.61–4.74
≥42.03175617.74.483.17–6.343.282.28–4.71
Total19 32718929.8    
Sum of skinfolds >90th percentile*
<22.625151134.51.00 1.00 
22.6–28.499788148.21.841.51–2.261.701.38–2.08
28.5–32.9425653112.52.932.38–3.622.421.95–3.01
33.0–37.4171426815.63.833.04–4.832.942.30–3.75
37.5–41.96078413.83.332.47–4.492.361.72–3.23
≥42.03195316.64.062.86–5.772.631.82–3.81
Total19 38918639.6    

Other outcomes

There was no relationship between maternal BMI and the occurrence of major congenital malformations or perinatal mortality, either in unadjusted or adjusted analyses.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References
  8. Supporting Information

In conjunction with the results of the primary analysis of the HAPO study,14 this report demonstrates that both maternal BMI and glycaemia have strong, independent associations with a range of clinically important pregnancy outcomes. Neither the participant’s glycaemic status nor her BMI was a target for active therapy in the HAPO cohort, removing a major source of potential confounding that was present in previous reports.

Parental size and infant birthweight are related through genetic and environmental mechanisms, with the stronger relationship between maternal and fetal size believed to represent contributions of the intrauterine environment.18 Our results demonstrate that increasing maternal BMI contributes to this effect independently of variations in glycaemic exposure. In addition to the strong relationship with birthweight >90th percentile, maternal BMI was also strongly related to fetal adiposity and hyperinsulinaemia, even after adjustment for maternal glycaemia. This highlights the potential importance of other nutrients including triglycerides, free fatty acids and amino acids19 and potentially of total caloric intake. Fetal beta cells may have been ‘programmed’ to increase insulin secretion much earlier in pregnancy before the OGTT.20 Other factors associated with maternal obesity, such as altered concentrations of adipocytokines and other inflammatory markers and changes in physical activity, may also play a role in determining fetal size and adiposity.21

Associations with fetal growth tended to plateau at the highest levels of maternal BMI, suggesting that a maximal influence is present before the mother’s BMI is categorised as class II or III obesity or that other factors limiting growth (e.g. increased frequency of pre-eclampsia) come into play at very high levels of BMI. Similar findings have been reported in obese and severely obese women diagnosed with gestational diabetes.22

Increased frequency of caesarean section with increasing BMI has been noted in previous reports.3,23 This was confirmed in HAPO and found to be independently related to BMI and glycaemic status. This has important practical and health economic implications24 in view of the rising prevalence of obesity in women of child-bearing age.25,26

Maternal BMI was strongly associated with both increased fetal size and adiposity and with pre-eclampsia. This is a paradox because pre-eclampsia is expected to be associated with smaller babies. However, only a small proportion of pregnancies in all BMI categories except the two highest categories developed pre-eclampsia. Other factors, such as substrate supply, may be dominant determinants of fetal growth in the overweight or obese gravida. The plateau in fetal size noted at the highest levels of maternal BMI may partly reflect the higher prevalence of pre-eclampsia in these women as 20–30% of these groups experienced pre-eclampsia.

Preterm delivery was less frequent with higher BMI, consistent with other large studies.3,10 This contrasts with the greater risk of preterm delivery that was found with rising plasma glucose14 and again suggests that alternative mechanisms contribute to pregnancy complications of obesity. The greater frequency of pre-eclampsia might have been predicted to lead to higher rates of preterm delivery in more obese women, but among HAPO participants pre-eclampsia occurred in only 11.7% of those with preterm delivery.

Maternal BMI was not associated with malformations or perinatal mortality, but HAPO was not designed or powered to examine such relationships. Although presence of a known congenital anomaly was not an exclusion criterion, it is unlikely that such women would have been recruited.

Major strengths of the HAPO data include careful and standardised documentation of maternal characteristics and pregnancy outcomes in a large, multinational, ethnically diverse group of pregnant women. Strict blinding of maternal glycaemia and the absence of any specific interventions within the study population help to reduce bias.

Some limitations should be noted. We hypothesise that maternal BMI influences pregnancy outcomes primarily through its association with maternal adiposity. However, BMI measured in the third trimester is less closely correlated with maternal fat mass than BMI measured in early pregnancy.27 Lean mothers tend to gain more (and obese mothers less) weight from the pre-pregnancy period to the third trimester (in accordance with current guidelines).28 This observation, confirmed in the HAPO cohort, may have attenuated differences between BMI groups but would not produce false-positive findings. Maternal height and weight were not blinded to participants or their caregivers. This may have influenced clinical decisions, especially regarding mode of delivery. A number of other factors that we did not evaluate in the HAPO Study, for example dietary intake (both calories and nutrient composition), physical activity and/or maternal weight gain are also potentially associated with birthweight, fetal adiposity and hyperinsulinaemia. A single measure of fasting plasma glucose at about 28 weeks of gestation is not a complete surrogate for time and dietary-related integrated glycaemia; however, it is strongly and independently related to the primary and most of the secondary outcomes of the HAPO Study.14,15

We cannot conclude that there is a causal relationship between increased maternal BMI and the adverse outcomes reported, although this would be consistent with both the current data and previous reports.8 We are unable to offer a pathophysiological explanation for the relationship between increased BMI and adverse pregnancy outcomes, but can conclude that mechanisms separate from, or in addition to, the association of hyperglycaemia and obesity are likely to play a role. Differences in supply of other substrates, as well as the inflammatory state associated with increased BMI, may contribute to adverse pregnancy outcomes.

Despite the clear association of obesity with adverse pregnancy outcomes, a recent systematic review has noted that there is minimal evidence to support any specific treatment strategy.29 One small randomised controlled trial has shown that caloric restriction during pregnancy can reduce weight gain and markers of insulin resistance, but reported no difference in major pregnancy outcomes.30

In conclusion, higher maternal BMI is strongly related to adverse pregnancy outcomes, especially those related to fetal size, adiposity and hyperinsulinism and to pre-eclampsia. It is, however, inversely related to premature delivery. These associations are independent of maternal glycaemia and other potential confounding factors. The pathophysiological mechanisms linking obesity to adverse pregnancy outcomes remain poorly defined and warrant further research. Increasing prevalence of maternal obesity in many countries will place an increasing burden on healthcare resources. Prevention and optimal treatment of maternal obesity have the potential to substantially reduce adverse fetal and maternal pregnancy outcomes.

Disclosure of interests

None of the members of the Writing Group report a conflict of interest.

Contribution to authorship

These individuals formed the Writing Group: HD McIntyre, BE Metzger, LP Lowe, AR Dyer, P Catalano, ER Trimble, B Persson, JJN Oats, M Hod, DR Hadden, DR Coustan.

Members of the HAPO Study Cooperative Research Group are listed in the Supporting Information Appendix S1.

Details of ethics approval

The Clinical Coordinating Center for this study undergoes annual ethics approval by the Northwestern University Office for the Protection of Research Subjects as Protocol # 0353-001. On 17 April 2009 it was approved for the period 8 May 2009 through to 7 May 2010.

Funding

The study was funded by grants R01-HD34242 and R01-HD34243 from the National Institute of Child Health and Human Development and the National Institute of Diabetes, Digestive, and Kidney Diseases, by the National Center for Research Resources (M01-RR00048, M01-RR00080), and by the American Diabetes Association. Support has also been provided to local field centres by Diabetes UK (RD04/0002756), Kaiser Permanente Medical Center, KK Women’s and Children’s Hospital, Mater Mother’s Hospital, Novo Nordisk, the Myre Sim Fund of the Royal College of Physicians of Edinburgh, and the Howard and Carol Bernick Family Foundation.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References
  8. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. References
  8. Supporting Information

Appendix S1. List of Authors.

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FilenameFormatSizeDescription
BJO_2486_sm_AppendixS1.doc27KSupporting info item

Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.