Introduction

Prevalence of obesity among children and adults in the United States has increased rapidly during the past decade (1,2). According to the most recent data from 1999 to 2000 in the United States, one-third of adults, 15.3% of children 6 to 11 years of age, and 15.5% of adolescents 12 to 19 years of age were obese (1,2). Childhood overweight is a strong predictor of adult obesity (3) and other adverse health consequences, especially type 2 diabetes and cardiovascular disease in adolescence and adult life (4,5). It has been proposed that fetal life, adiposity rebound, and adolescence are three critical periods for the development of obesity (6). Previous studies have shown that maternal gestational diabetes (7), birth weight (7,8), breast-feeding, and early childhood growth (9,10,11) have been associated with the development of overweight in childhood and adolescence. Associations of prenatal and early childhood influences with obesity later in life have led to the hypothesis of "fetal origins of obesity" (12,13).

Maternal prepregnancy obesity is associated with an increased risk of adverse pregnancy-related outcomes such as late fetal death and birth defects (14). Few studies have examined maternal prepregnancy obesity as a long-term risk factor for childhood overweight among their offspring. In a previous study, mothers of obese children tended to be obese before their pregnancy (15). Recent studies have shown that maternal concurrent obesity is more highly correlated with childhood obesity than prenatal factors such as birth weight (3,16). A recent study has shown that maternal obesity in early pregnancy more than doubles the risk of obesity at 2 to 4 years of age (17). Nevertheless, most previous studies focused on maternal obesity during or after pregnancy rather than maternal prepregnancy obesity, which may be a more reliable indicator of maternal impact on offspring overweight status, representing both shared genes and shared prenatal and postnatal environmental factors within families (3,12,13).

Current literature on the impact of breast-feeding on overweight in childhood is inconclusive. Some studies have shown a significant protective effect (9,10,18,19,20), whereas others have shown a weak effect or no effect (21,22,23). The inconsistent results in previous studies suggest that the association between breast-feeding and childhood overweight may be modified by one or more extraneous variables. It is unclear whether breast-feeding predicts childhood overweight independently of maternal prepregnancy obesity. In addition, it is not known whether the actual joint effects of maternal prepregnancy obesity and lack of breast-feeding may exceed their combined independent effects on childhood overweight.

Therefore, the objective of this study was to assess the independent effects of maternal prepregnancy obesity and breast-feeding, as well as their additive and multiplicative interactions on childhood overweight, by using the 1996 child data of the National Longitudinal Survey of Youth (NLSY79)¹ in the United States. We hypothesized that maternal prepregnancy obesity would be associated with an increased risk, whereas breast-feeding would be associated with a decreased risk, of childhood overweight. We further hypothesized that the presence of both maternal prepregnancy obesity and lack of breast-feeding would exert a greater effect on childhood overweight than the combined independent effect of each single factor alone.

Research Methods and Procedures

Design and Sample

The NLSY79 is a nationally representative sample of 12,686 young men and women who were 14 to 22 years of age when first surveyed in 1979. Beginning in 1986, all children of NLSY79 women were surveyed with supplemental information on physical and behavioral development and health status collected biennially through 2000 (24). Sampling weights were constructed to adjust for differential non-response and oversampling of blacks, Hispanics, and economically disadvantaged non-black/non-Hispanic whites.

In this study, a sample of children 2 to 14 years of age in 1996 was analyzed. We excluded the following children from the original sample: <2 years of age (n = 433), with gestational age <32 weeks or with extreme birth weights (<0.5 or >6 kg) (n = 587), with incomplete data for height or weight (n = 149), and whose height or weight was reported by their mothers (n = 1626). Compared with children included in these analyses (n = 2636), those excluded from the analyses were similar in sex and ethnicity. The average number of siblings per household in the current sample was 1.6. Therefore, the non-independence of observations was unlikely to affect results in this study because the average household size of children was small.

Procedures

Computer-assisted personal interviewing, a research technique that uses portable computers or unmanned terminals to conduct personal interviews, was used to collect information from NLSY79 women and their children (24). Mother-child assessments were conducted at the same time at the main NLSY79 interview of each mother. Mothers' height, weight just before pregnancy, and weight just before the delivery of child were obtained by self-report at the first instance when their children were investigated. Information about the mother's health and prenatal care, including timing of first prenatal examination, extent of alcohol use, and smoking during pregnancy, was assessed. Children's characteristics at birth, such as birth weight, birth length, gestational age, and birth order, were reported by mothers. The duration of breast-feeding, measured as the weeks of age after birth when breast-feeding ended, was determined at the first interview when the children were investigated. For each child in the study, demographic information such as date of birth, age, sex, and race/ethnicity and characteristics of the child's immediate family, such as mother's age and highest education levels at childbirth, as well as annual family net income, were assessed. The overall response rate in the 1996 survey was >90%. Details of the various pre- and postnatal measures have been reported previously (25).

Definition of Childhood Overweight and Maternal Prepregnancy Obesity

Children's height and weight were measured by interviewers using a portable scale and a tape measure. Details of the protocol for the weight and height measures have been reported previously (26). BMI was calculated based on measured weight (kilograms) and height (meters) using the following formula: BMI = weight divided by height squared. BMI percentile for age and sex was derived using the Centers for Disease Control growth charts (27). Childhood overweight was defined as BMI 95th percentile. Maternal prepregnancy BMI was calculated based on self-reported height and weight just before pregnancy. Maternal obesity was defined as a BMI 30 kg/m², and overweight was defined as 25 BMI < 30 kg/m² (28).

Statistical Analyses

The primary outcome variable in this study was childhood overweight, which was dichotomized as 0 (nonoverweight, BMI < 95th percentile) and 1 (overweight, BMI 95th percentile). In the multiple logistic regression analyses of main effects, maternal prepregnancy BMI was categorized into three levels: 1 = normal (BMI < 25 kg/m²), 2 = overweight (25 BMI < 30 kg/m²), and 3 = obese (BMI 30 kg/m²). Breast-feeding was categorized into three levels: 1 = never breast-fed, 2 = breast-fed 1–3 months, and 3 = breast-fed 4 months. Maternal weight gain during pregnancy was calculated as the difference between mothers' weight just before the delivery of child and weight just before pregnancy. The SUDAAN, a statistical package designed specifically to analyze data from complex surveys that incorporate multistage sampling and unequally weighted designs (29), and SAS (30) were used for all analyses in this study.

The prevalence rates of childhood overweight stratified by each level of maternal prepregnancy BMI and breast-feeding duration were estimated for the total sample and by three age groups (2 to 6, 7 to 10, and 11 to 14 years of age, with an approximate tertile split for age). The odds ratios (ORs) and 95% confidence intervals (CIs) for the main effects of maternal prepregnancy BMI and breast-feeding on childhood overweight were assessed using multiple logistic regression models adjusted for the effects of potential confounding factors such as age, sex, ethnicity, gestational age, birth weight, birth order, maternal age at childbirth, maternal smoking, alcohol use, weight gain during pregnancy, maternal highest educational levels, and annual family net income. Linear (-1, 0, 1) and quadratic (1, -2, 1) trends of ORs for each categorical variable were assessed with the Wald F tests in the multiple logistic regression models by specifying the EFFECTS statements (29).

Interaction between two or more exposures implies that the observed joint effect on disease risk when the two or more exposures are present is greater (synergy) or smaller (antagonism) than the expected joint effect based on their independent effects alone (31,32,33,34,35,36,37). On the additive scale, the expected joint effect of two exposures is the addition of the independent effects of two exposures minus the baseline effect of exposure to neither, and on the multiplicative scale, the expected joint effect is the multiplication of the two independent effects divided by the baseline effect (32,33,34,35). If no interaction is present, the observed joint effect of two exposures on disease risk is equivalent to the expected joint effect. In contrast, if the observed joint effect of two exposures is significantly greater or smaller than the expected joint effect on the additive scale, an additive interaction is assumed. Similarly, if the observed joint effect of two exposures is significantly greater or smaller than the expected joint effect on the multiplicative scale, a multiplicative interaction is assumed (32,33,34,35). The Wald statistics (F or ² test), likelihood-ratio statistics, and score (Rao) statistics have been proposed to test for interaction on either additive or multiplicative scale (31,34,37). We used the Wald F tests available in SUDAAN to assess the interactions on both the additive and multiplicative scales in our study (29).

The reference group for OR estimations was determined as children whose mothers had normal prepregnancy BMI and who were breast-fed for 4 months. The analyses for main effects were performed for the total sample and the three age-specific subsamples (2 to 6, 7 to 10, and 11 to 14 years of age) separately. Because similar patterns in the associations between the major predictors and the outcome variable were observed, we combined all age groups to estimate and assess the interaction between maternal prepregnancy BMI and lack of breast-feeding on childhood overweight to ensure adequate statistical power (37).

Results

Demographic, Maternal, and Early Infancy Characteristics of the Sample

The unweighted sample size and weighted percentages of the sample for each characteristic of children are shown in Table 1. The weighted sample represented 51.3% boys, 78.0% white, 15.0% black, and 7.0% Hispanic children. Nearly 12% of children were at risk of overweight (85th BMI < 95th percentile), and 13% were overweight (BMI 95th percentile). In addition, 43.2% of children were never breast-fed, 33.5% were breast-fed for 1–3 months, and only 23.3% were breast-fed for 4 months.

Table 1. - Sample and population-weighted characteristics of children of the 1996 NLSY79.

Full table

The unweighted sample size and weighted percentages of the sample for each characteristic of mothers are shown in Table 2. Among mothers, 17.6% were overweight (25 BMI < 30 kg/m²) and 9.4% were obese (BMI 30 kg/m²) just before pregnancy. Nearly 30% of mothers reported ever smoking, and 40% reported ever using alcohol during pregnancy. Mean maternal weight gain during pregnancy was 14.5 7.6 kg (SD).

Table 2. - Sample and population-weighted characteristics of mothers of the 1996 NLSY79.

Full table

Prevalence of Overweight

The weighted prevalence of childhood overweight was higher among children whose mothers were overweight or obese before pregnancy than among those whose mothers had normal BMI (Figure 1). The prevalence of overweight was lower among children who were ever breast-fed than among those who were never breast-fed at each level of maternal prepregnancy BMI. However, there was a clear pattern of additive interactions between maternal prepregnancy BMI and breast-feeding on childhood overweight prevalence. The highest prevalence of childhood overweight was observed among children whose mothers were obese and who were never breast-fed (31.5%), whereas the lowest prevalence of overweight was observed among children whose mothers had normal BMI and who were breast-fed 4 months (6.0%; Figure 1A). Similar patterns of prevalence of childhood overweight were observed in children in each of the three age groups (Figure 1 B-D).

Figure 1.

Prevalence of childhood overweight (BMI 95th percentile) by maternal prepregnancy BMI and breast-feeding for the total sample (A: 2 to 14 years of age) and by three age groups (B: 2 to 6 years of age; C: 7 to 10 years of age; D: 11 to 14 years of age). 1996 NLSY79 Child and Young Adult Survey, United States.

Full figure and legend (139K)

Main Effects

The ORs and linear and quadratic trends of childhood overweight in relation to maternal prepregnancy BMI and breast-feeding for the total sample are shown in Table 3. In Model 1, ORs for maternal prepregnancy BMI and breast-feeding were adjusted for age, sex, ethnicity, gestational age, birth weight, birth order, maternal smoking, alcohol use, weight gain during pregnancy, maternal age, highest education levels at childbirth, and annual family net income, but were not adjusted for each other. We found that children whose mothers were overweight (adjusted OR = 2.4; 95% CI, 1.7, 3.4) or obese (OR = 3.6; 95% CI, 2.4, 5.5) were more likely to be overweight compared with those whose mothers had normal BMI. In contrast, children who were breast-fed for 1 to 3 months (OR = 0.7; 95% CI, 0.5, 0.9) and 4 months (OR = 0.6; 95% CI, 0.4, 0.9) were less likely to be overweight.

Table 3. - Linear and quadratic effects of maternal prepregnancy BMI and breast-feeding on overweight (BMI 95th percentile) for children 2 to 14 years of age, the 1996 NLSY79, total sample.

Full table

Thereafter, in Model 2, we further adjusted the ORs for maternal prepregnancy BMI and breast-feeding simultaneously in addition to the covariates in Model 1. After the additional adjustment, the effect of maternal prepregnancy BMI was slightly increased, whereas the effect of breast-feeding was slightly attenuated. The changes in the magnitude of the effects of maternal prepregnancy BMI or breast-feeding on childhood overweight risk indicated that the association of breast-feeding and childhood overweight may be partially confounded by maternal prepregnancy BMI and vice versa. However, the persistent significance of both maternal prepregnancy BMI and breast-feeding further showed their independent effects on childhood overweight.

Model 2 was repeated for the three age-specific subsamples, and similar patterns of ORs were yielded (Table 4) compared with the total sample. It seemed that the association between maternal prepregnancy obesity (BMI 30 kg/m²) and childhood overweight was slightly stronger in older children (7 to 10 years of age: OR, 5.7; 95% CI, 2.9, 11.7; 11 to 14 years of age: OR, 4.3; 95% CI, 1.7, 10.8) compared with younger children (2 to 6 years of age: OR, 2.8; 95% CI, 1.3, 6.0). The association between breast-feeding and childhood overweight was similar across the three age groups. Because of reduced sample size for each age group, the 95% CIs of ORs for both maternal prepregnancy BMI and breast-feeding were wider than those for the total sample (Table 3).

Table 4. - Linear and quadratic effects of maternal prepregnancy BMI and breast-feeding on overweight (BMI 95th percentile) for children 2 to 14 years of age, the 1996 NLSY79, by age group.

Full table

Linear and Quadratic Trends

For the total sample, there was a significant linear trend (or dose-response relationship) for the effects of maternal prepregnancy BMI on childhood overweight (p < 0.001), indicating that a higher level of maternal prepregnancy BMI was associated with an increased risk of childhood overweight (Table 3). In addition, there was a significant linear trend for the effect of breast-feeding on childhood overweight (p = 0.02), indicating that a longer duration of breast-feeding was associated with a reduced risk of childhood overweight. The linear trend for the effect of breast-feeding on childhood overweight seemed to be slightly attenuated after adjusting for maternal prepregnancy BMI (p = 0.02 vs. p = 0.06). No quadratic trends of maternal prepregnancy BMI and breast-feeding were detected in this study.

Additive and Multiplicative Interactions

The adjusted ORs of childhood overweight stratified by the levels of maternal prepregnancy BMI and breast-feeding are shown in Table 5. According to the independent effect of maternal prepregnancy obesity (OR, 3.5; 95% CI, 1.2, 10.0) and the independent effect of never being breast-fed (OR, 1.7; 95% CI, 0.9, 3.2) on childhood overweight, the expected joint effect of maternal prepregnancy BMI and lack of breast-feeding was 4.2 (3.5 + 1.7 –1 = 4.2) on the additive scale and 6.0 (3.5 1.7 ÷ 1 = 6.0) on the multiplicative scale. Therefore, the observed joint effect when both exposures were present (OR, 6.1; 95% CI, 2.9, 13.1) was greater than the expected joint effect on the additive scale (OR, 4.2) but equivalent to the expected joint effect on the multiplicative scale (OR, 6.0), indicating a possible interaction between maternal prepregnancy BMI and lack of breast-feeding on the additive scale and no evidence of interaction on the multiplicative scale. The Wald F test further showed that the additive interaction between maternal prepregnancy BMI and lack of breast-feeding was statistically significant (Wald F, 3.1; df, 4; p < 0.05), but the multiplicative interaction was not statistically significant (Wald F, 0.31; df, 4; p = 0.87).

Table 5. - OR and 95% CI of overweight (BMI 95th percentile) in relation to maternal prepregnancy BMI and breast-feeding for children 2 to 14 years, the 1996 NLSY79 Child Survey, total sample.

Full table

Discussion

Findings from our study indicated that higher maternal prepregnancy BMI was strongly associated with an increased risk, whereas breast-feeding was associated with a decreased risk of childhood overweight. The effect of breast-feeding on childhood overweight was slightly attenuated after adjusting for maternal prepregnancy BMI. A significant additive interaction between maternal prepregnancy BMI and lack of breast-feeding was identified in our study, such that children whose mothers were obese before pregnancy and who were never breast-fed were at a 6-fold greater risk of being overweight during childhood compared with children whose mother had normal BMI before pregnancy and who were breast-fed for at least 4 months. The magnitude of risk for childhood overweight seemed to decline with increasing duration of breast-feeding, independently of maternal prepregnancy BMI, but the protective effect of breast-feeding seemed, in this study, to be most significant among children whose mothers had normal prepregnancy BMI.

Maternal prepregnancy obesity was the strongest predictor of childhood overweight, which is consistent with previous studies (3,17,38). It seems that this risk association may emerge at an early age and persist in the older age groups, as shown in our study. The effect of maternal prepregnancy obesity on childhood overweight may be mediated through maternal glucose intolerance or gestational diabetes (7,39). On the other hand, maternal prepregnancy obesity has been associated with increased birth weights of children (14,40), which may, in turn, lead to childhood overweight. However, this mediated effect seems to be weak (7,40). Thus, it is likely that maternal prepregnancy obesity affects childhood overweight through shared genetics or pre- and postnatal environmental factors, as shown in prospective and twin studies (3,13,17,41).

One of the important findings in our study was that the protective effect of breast-feeding against childhood overweight seemed to be consistent regardless of maternal prepregnancy BMI. In particular, a significant protective effect of breast-feeding was detected among children whose mothers had normal prepregnancy BMI. Although the overall risk of being overweight was high among children whose mothers were overweight or obese, breast-feeding seemed to reduce the magnitude of risk of being overweight during childhood. In previous studies, the protective effect of breast-feeding against childhood obesity has been inconsistent (9,10,18,19,21,22,23). The disagreement in previous research may be a consequence of several limitations such as varying definition of breast-feeding, different age periods of life course, and lack of adjustment for additional possible confounders. Most importantly, the association between breast-feeding and childhood overweight may be modified by other prenatal factors. Maternal prepregnancy obesity seems to be a significant effect modifier in the association between breast-feeding and childhood overweight on the additive scale, as shown in this study.

Our study and previous studies suggest that the effect of breast-feeding may be more obvious in combination with maternal nutritional, behavioral, and physical status before or during pregnancy. Nevertheless, the effect of breast-feeding on childhood overweight needs to be further studied because little is known about the significance of exclusivity and duration of breast-feeding in different children. Therefore, while some breast-feeding seems to be protective, the crafting of accurate health policies will require more substantive research in this area. Our findings have significant implications in clinical practice and prevention. As shown in a recent study (42), women who were obese before pregnancy were less likely to initiate and maintain breast-feeding. Because there seems to be a significant additive interaction between maternal obesity before pregnancy and lack of breast-feeding on childhood overweight in the long run, intervention may be needed among women who are obese before pregnancy to facilitate initiation and maintenance of breast-feeding to reduce offspring's risk of overweight during childhood.

It is noteworthy that the popularity of multiple regression methods based on multiplicative models has frequently led to equating interaction exclusively with multiplicative interaction. However, additive interaction may be of greater interest if the prevention of health events (e.g., childhood overweight) is being considered. From the viewpoint of translating epidemiological findings into clinical and public health practice, presence of additive interaction is imperative, even if multiplicative interaction is absent (32,33).

There are several strengths in this study. First, we used a nationally representative cohort to test our hypothesis, thus increasing the external validity of our findings. Second, maternal prepregnancy BMI was used as a potential predictor for childhood overweight because it was a more valid and reliable indicator of maternal influences in the prenatal environment. Finally, we used an innovative approach to examine the additive interaction of maternal prepregnancy BMI and breast-feeding on childhood overweight, while adjusting for the possible confounding effects of maternal weight gain and smoking during pregnancy, gestational age, birth weight, and many other potentially important biological and socio-demographic factors.

Two limitations are present in this study. First, we did not have information on diet or physical activity in childhood, which could possibly confound or moderate the relationship between early life influences and childhood overweight. Second, maternal weight and height were self-reported. However, self-reported measures of height and weight in adults have been shown to be valid and reliable for identifying associations in epidemiological studies (43,44). In addition, maternally reported birth weights, gestational age, and other neonatal events have been shown to be sufficiently accurate for clinical and epidemiological research (45).

In conclusion, the findings from our research suggest that a combination of early life risk factors, such as maternal prepregnancy overweight or obesity and lack of breast-feeding, may significantly contribute to the increased risk of childhood overweight. These are potentially modifiable risk factors that could be targeted in the prevention of childhood overweight. Future research is needed to help us understand the mechanisms by which these early life factors affect subsequent risk for childhood overweight.

Notes

¹ Nonstandard abbreviations: NLSY79, National Longitudinal Survey of Youth 1979; OR, odds ratio; CI, confidence interval.

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Acknowledgments

We thank Dr. Michael I. Goran for constructive comments on the first version of this paper. Dr. Li is partially supported by research grants from the American Heart Association (AHA 0460043Z) and the National Institute of Health/National Cancer Institute (R24 CA95835–01).