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Original Research

Correlation Between Birth Weight and Maternal Body Composition

Kent, Etaoin MRCOG, MRCPI; O’Dwyer, Vicky MRCPI; Fattah, Chro MD; Farah, Nadine MD; O’Connor, Clare MRCPI; Turner, Michael J. FRCOG, FRCPI

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doi: http://10.1097/AOG.0b013e31827a0052
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Birth weight is an important obstetric outcome. Both low and high birth weight are associated with increased obstetric and neonatal complications in the short-term, and with an increased risk of cardiovascular disease and metabolic syndrome later in life.1,2

Epidemiologic studies have linked birth weight with the mother's body composition with maternal obesity reportedly associated with an increased risk of fetal macrosomia and neonatal adiposity.2,3 In a report from the Institute of Medicine, gestational weight gain was associated with both restricted and excessive intrauterine fetal growth.4 Concerns about the affect of rising levels of maternal obesity on intrauterine fetal growth led the Institute in 2009 to publish revised guidelines recommending lower levels of gestational weight gain for obese women.5

However, the evidence linking maternal obesity and birth weight is problematic. Maternal obesity is usually classified on the basis of a body mass index (BMI, calculated as weight (kg)/[height (m)]2) of 30 or higher. Most epidemiologic studies are retrospective and based on self-reporting of weight and height to calculate BMI, which under-reports obesity levels and often lead to BMI miscategorization.6–8 Furthermore, BMI does not measure fat directly but acts as a surrogate marker of adiposity.

Fat mass and fat-free mass in adults can be measured directly using bioelectric impedance analysis, which is a safe, noninvasive, and convenient method to determine body composition.9–12 Recent technical advances with eight-electrode multifrequency bioelectric impedance analysis (multifrequency bioelectric impedance analysis) mean that bioelectric impedance analysis can be applied in large numbers of patients to analyze body composition and also to analyze the distribution of fat and fat-free mass without recourse to the supine position.10–12 We have demonstrated previously that in early pregnancy the use of multifrequency bioelectric impedance analysis is feasible and reproducible, and that it correlates strongly with clinical and endocrine markers of maternal adiposity.11,12 The purpose of this prospective observational study was to estimate whether maternal fat mass measured in the first trimester correlated better with birth weight than did fat-free mass, and to identify which maternal body composition parameters correlated best with birth weight.


The study was performed prospectively between July 2008 and December 2011, in a large university teaching hospital. The study was approved by the Ethics Committee of the Coombe Women and Infants University Hospital and all participants gave written informed consent. It is hospital policy to offer a dating ultrasound scan to all women in early pregnancy. Women were recruited after ultrasonographic confirmation of a viable, singleton pregnancy in the first trimester. To avoid ethnic confounding variables, the study was confined to Caucasian women. Women who were younger than age 18 years or who were unable to give informed consent were excluded. Women were also excluded if they had pre-existing diabetes mellitus.

Height was measured using a wall-mounted digital meter stick to the nearest 0.1 cm with the woman standing erect in her bare feet. Weight was measured digitally to the nearest 0.1 kg with the woman in light clothing. An allowance of 0.5 kg was made for clothing and the BMI was calculated. Bioelectric impedance analysis was performed using the Tanita MF-180CA with the woman in her bare feet and wearing light clothing. Segmental bioelectric impedance analysis of the trunk and individual limbs, along with whole-body analysis, were performed on each patient.

Clinical and sociodemographic details were recorded and the women were discharged back to their own obstetric team for the management of the pregnancy and delivery. The hospital has a policy of selective and not universal screening for gestational diabetes mellitus.13 The antenatal and delivery details were obtained postpartum from the hospital’s computerized database.

To identify predictors of birth weight, univariable correlations of birth weight with maternal demographic, anthropometric, and clinical variables, bioelectric impedance analysis measures of fat mass and fat-free mass, and gestational age at delivery were assessed by Pearson or Spearman correlation coefficients according to distribution. Variables thus identified as having a statistically significant relationship with birth weight were incorporated into a multivariable linear regression model, with birth weight as the dependent variable.

Multiple logistic regression analysis was then performed to generate odds ratios for birth weight greater than 4 kg per bioelectric impedance analysis--measured fat-free mass quartile, with the lowest quartile serving as the reference group. Models incorporated maternal demographic and clinical variables, along with gestational age at delivery and bioelectric impedance analysis--measured fat mass. A similar analysis was performed to estimate the effect of fat mass on odds of birth weight greater than 4 kg, this time adjusted for fat-free mass.

Statistical analyses were performed using SPSS 18.0 statistical software. P<.05 was considered statistically significant.


Of the 3,000 women recruited in the first trimester, 320 delivered before 37 weeks of gestation and 62 (2.1%) had gestational diabetes mellitus diagnosed on targeted screening. Because gestational age and gestational diabetes mellitus are both strong and independent determinants of birth weight, these women were excluded from further analysis. The characteristics of the remaining 2,618 women are shown in Table 1. The mean BMI of the study population was 25.4 (standard deviation 5.1), with 43.9% of patients classified as overweight or obese.

Table 1
Table 1:
Characteristics of Study Population

The association between birth weight and maternal BMI, fat-free mass, fat mass, and visceral fat level was evaluated, and in a univariable analysis all four variables were significant predictors of birth weight (Table 2). The variable that showed the strongest correlation with birth weight was maternal fat-free mass. The analysis was repeated for fat mass and fat-free mass of arms, legs, and trunk, and in all cases both parameters were associated with birth weight (P<.001).

Table 2
Table 2:
Univariate Correlations With Birth Weight (N=2,618)

A multiple linear regression model was built using variables with significant univariable relationships with birth weight. After adjustment for age, parity, gestational age at delivery, smoking status, and fat mass, fat-free mass remained a significant predictor of birth weight (model R2=0.254, standardized β=0.237; P<.001) (Table 3). In contrast, no relationship was found between fat mass and birth weight using this model.

Table 3
Table 3:
Multivariate Regression Analysis of Predictors of Birth Weight

Compared with women in the first fat-free mass quartile, the unadjusted odds ratios for birth weight more than 4 kg for women in the second, third, and fourth fat-free mass quartiles were 1.87 (95% confidence interval [CI] 1.26–2.76), 2.38 (95% CI 1.63–3.48), and 4.89 (95% CI 3.43–6.98), respectively (P<.001) (Table 4). After adjustment for age, parity, gestational age at delivery, and maternal fat mass, fat-free mass remained a predictor of birth weight more than 4 kg, with women in the highest fat-free mass quartile having an odds ratio of 3.64 (95% CI 2.34–5.68) for birth weight greater than 4 kg. In contrast, no relationship was seen between odds of birth weight more than 4 kg and maternal fat mass after adjustment for fat-free mass (Table 4).

Table 4
Table 4:
Crude and Adjusted Odds Ratios of Fat-Free Mass and Fat Mass Quartiles as Predictors of Birth Weight Greater Than 4 kg


In a large, prospective, observational study of women who delivered singleton neonates at term, we found that birth weight correlated positively with measurement of maternal fat-free mass, and not fat mass, in the first trimester of pregnancy. The relationship between birth weight and different biological and psychosocial variables is complex. However, recent studies have often focused on the increased risk of fetal macrosomia with maternal obesity.14 Concerns about aberrant fetal growth, albeit based on limited evidence, influenced the Institute of Medicine’s decision to revise downwards the recommendations on gestational weight gain for obese women.5

Although BMI, measured accurately, is an excellent surrogate marker of adiposity, it provides no information on the distribution of either fat or fat-free mass. This study has used bioelectric impedance analysis to directly measure maternal body composition, which means fat and fat-free mass have been measured and also their distributions in early pregnancy have been measured. Strengths of this study include the accurate dating of all pregnancies by first-trimester ultrasonography; although gestational age at delivery is a strong predictor of birth weight, few studies confirm the dates with an early ultrasound scan.15 Also, all patients had BMI calculated after digital measurement of height and weight. This contrasts with other studies in which BMI often was calculated using self-reporting of weight and height, which is epidemiologically misleading.8 Confining the study to Caucasian women without gestational diabetes mellitus and with a singleton pregnancy who delivered at term removes other important confounding variables for birth weight.

A potential weakness in our study is that recruitment was by convenience and was not consecutive. The analysis is also based on proprietary formulae, which were calculated for American and European women. In view of ethnic differences in adiposity, the findings from this study may not be applicable to other ethnic groups.16

Our findings are consistent with those of previous smaller studies using bioelectric impedance analysis in pregnant women. In an Italian longitudinal study, single-frequency tetrapolar bioelectric impedance analysis evaluation was first conducted at 15–17 weeks of gestation and repeated at 20–22 weeks, 25–27 weeks, 30–32 weeks, and 35–37 weeks of gestation.17 Body water in the second trimester, but not in the third trimester, was predictive of birth weight. In a smaller study of 29 women near term using single-frequency bioelectric impedance analysis after 36 weeks of gestation, fat-free mass was the most important maternal body component correlating with birth weight.18 In a study of 169 women who delivered a singleton pregnancy at term, maternal body composition was measured postpartum using bioelectric impedance analysis. Fat-free mass and total body water explained the major proportion of the birth weight.19

In a prospective study of 200 healthy women in New York who delivered at term, body fat was determined by a complex multicompartment model between 12 and 16 weeks of gestation, and again after 36 weeks of gestation.20 After regression modeling, maternal body water, but not body fat, correlated positively with birth weight.

These scientific observations are supported by larger epidemiologic reports. Despite increasing levels of maternal obesity in recent years, there has been no increase in the incidence of fetal macrosomia (birth weight more than 4.5 kg) in our own hospital,21 nationally,22 or in other countries such as the United States.23

It is notable that in our study maternal adiposity was measured in the first trimester and that women with pre-existing diabetes mellitus were excluded. Maternal obesity is an important risk factor for the development of gestational diabetes mellitus, particularly moderate to severe obesity. Thus, any epidemiologic association between maternal obesity and increased fetal growth may be the result of metabolic abnormalities associated with gestational diabetes mellitus, such as hyperglycemia and hypertriglyceridemia, rather than maternal obesity itself.

Our findings have important clinical implications. The Institute of Medicine in 2009 produced revised guidelines that lowered the recommended gestational weight gain for obese women to 5–9 kg.4 The number of obese women with a gestational weight gain exceeding the recommendations is high, but obese women still gain less weight during pregnancy than nonobese women.24

There is a potential risk that overzealous attempts to reduce gestational weight gain in obese women may cause complications by reducing calorie intake or micronutrients. This reduction may be detrimental to the fetus and, in the absence of gestational diabetes mellitus, our results suggest minimizing maternal weight gain may not prevent fetal macrosomia and could potentially increase the risk of intrauterine growth restriction.


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© 2013 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.