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Division of Nutritional Sciences, Cornell University, Ithaca, NY and * Mary Imogene Bassett Hospital and Research Institute, Cooperstown, NY
3 To whom correspondence should be addressed. E-mail: kmr5{at}cornell.edu.
ABSTRACT
High prepregnant BMI is associated with reduced initiation and duration of breast-feeding (BF). To examine how gestational weight gain (GWG) might modify this association, over a 9-y period, we identified all women (n = 2783) who had attempted to breast-feed their newborns. From their medical records, we categorized them by prepregnant BMI [as underweight (<19.8 kg/m2), normal-weight (19.8–26.0 kg/m2), overweight (26.1–29.0 kg/m2) or obese ( >29.0 kg/m2)] and GWG [as below, within, or above the amount recommended by the Institute of Medicine]. Women with a normal BMI who gained within these recommendations served as the reference group in regression analyses, which were adjusted for confounding factors. Both normal-weight (P < 0.05) and obese (P < 0.01) women who exceeded the recommended GWG had higher odds of failing to initiate BF (defined as continuing to breast-feed at 4 d postpartum). Underweight (P < 0.05), overweight (P < 0.05), and obese (P < 0.01) women who exceeded the recommendations for GWG as well as obese women who gained within the recommendations (P < 0.01) had a higher risk of early discontinuation of exclusive BF. Only obese women who gained within or exceeded the recommendations (P < 0.01) for GWG had a higher risk of early discontinuation of any BF. Excessive GWG was associated with a measure of failure to initiate and/or sustain BF in all categories of prepregnant BMI. Thus, in addition to conceiving at a healthy weight, gaining the recommended amount of weight during pregnancy is also important for successful BF.
KEY WORDS: • breast-feeding • gestational weight gain • lactation • obesity • pregnancy
Women of reproductive age have not escaped our national epidemic of obesity (1). Excessive weight at the time of conception is not only a problem for women's health during gestation and at the time of delivery, but it is also a problem after birth. In particular, we reported that among White (2) and Hispanic (3), but not Black (3) women who ever breast-fed their babies, those who were overweight or obese were more likely to cease breast-feeding (BF)4 by the time of hospital discharge. Moreover, among those who continued to breast-feed their babies, overweight and obese women ceased BF sooner than normal-weight women. Similar findings were reported by others (4–9).
This association between high prepregnant BMI and poor lactational performance likely has many causes. These include the mechanical difficulties associated with proper positioning of the infant when the mother is too heavy and also delayed onset of copious milk secretion (lactogenesis II, which normally occurs at 
3 d postpartum) (10,11). We showed recently that overweight and obese women do not produce as much prolactin in response to suckling in wk 1 after delivery as normal-weight women (12). Although it is possible that psychosocial factors, such as perceived self-efficacy for BF, might also contribute to the association between high prepregnant BMI and poor lactational performance, we were not able to detect this (11).
In most of the studies of the association between prepregnant BMI and poor lactational performance, prepregnant BMI was used to capture the woman's relative fatness at delivery, ignoring the potentially large contribution of gestational weight gain (GWG) to her fatness at this time. This is both a scientific and a practical issue. It is unknown whether maternal fatness before conception, gain in fatness during pregnancy, or a combination of these (fatness at delivery) is important in the etiology of failure to initiate and sustain BF. Moreover, if GWG were to modify the association between high prepregnant BMI and lactational performance, helping women to achieve an appropriate GWG could potentially help them improve their ability to breast-feed their babies.
In the investigation reported here, we examined whether GWG was independently associated with the initiation and continuation of BF and whether GWG modified the previously observed association between high prepregnant BMI and these outcomes. We included only women who ever breast-fed the baby and investigated the initiation of BF as well as the duration of exclusive (EBF) and any BF (ABF). To enhance the relevance of our findings to public health practice, we categorized GWG according to the Institute of Medicine (IOM) recommendations.
MATERIALS AND METHODS
The study was conducted as a review of medical records at Mary Imogene Bassett Hospital in Cooperstown, NY and its associated regional clinics in 3 nearby counties. It was approved by the Cornell University Committee on Human Subjects as well as the Institutional Review Board at Bassett Hospital.
Selection criteria. We expanded our previous review of medical records (8) to include births from January 1988 through December 1997. We limited our study population to those mother-infant dyads in which BF was ever attempted and to those thought to have a low potential for contraindications to BF. Mothers also had to be 19–49 y old and free of gestational diabetes (Fig. 1). From among 3802 live singleton births we identified during the study period, 2957 dyads met these criteria and were eligible for the study. Of the eligible mother-infant dyads, 2783 had information available for all key variables and were included in the analyses presented here.
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Information on the mother's self-reported prepregnancy weight and measured height, mother's measured weight at delivery or at the last clinic visit before delivery, method of delivery (cesarean or vaginal), parity (nulliparous or parous), duration of hospital stay, gestational diabetes mellitus, education and age, participation in the Special Supplemental Nutrition Program for Women, Infants and Children (WIC) and the Prenatal Care Assistance Program (PCAP, a program for low-income pregnant women in New York State), maternal smoking at the time of conception, and the infant's birth weight and gestational age (assessed by the date of the mother's last menstrual period or by ultrasound during pregnancy) were also collected from the clinic records. Mother's BMI (weight/height2, kg/m2) was calculated using prepregnant weight and height. Women were categorized as underweight (BMI <19.8 kg/m2), normal weight (19.8–26.0 kg/m2), overweight (26.1–29.0 kg/m2), or obese (>29.0 kg/m2) according to the IOM (13).
Gestational weight gain was calculated as the difference between the mother's prepregnant weight and her last weight before delivery. This weight was obtained within 7 d before delivery. Some women (n = 25) in the sample lost weight during pregnancy. Because their weight loss did not exceed –4 SD from the mean pregnancy weight change, they were retained in the analysis. GWG was categorized as being less than, within, or greater than the IOM guidelines (13).
Data analysis. ANOVA with post hoc contrasts or logistic regression analysis (for outcomes that were dichotomous) was used to compare the characteristics of women who were classified as underweight, normal-weight, overweight, or obese at conception and among women gaining less than, within, or more than the IOM recommendations. Pairwise median tests were used to compare the duration of EBF or ABF among the categories of prepregnant BMI and also among the categories of GWG. Student's t tests were used to compare characteristics of women who successfully initiated BF with those who did not. All differences were considered significant at P < 0.05. Analyses were conducted using STATA 6.0 software.
Analysis of initiation of BF. The combined effects of prepregnant BMI and GWG on initiation of BF were analyzed with logistic regression, modeling the odds of unsuccessful initiation. All 2375 women who had attempted to breast-feed at delivery (ever gave their infants a chance to suckle), had a value for prepregnant BMI and GWG, and had no missing data for covariates were included in the analysis. Women were classified into 12 mutually exclusive categories of prepregnant BMI (<19.8, 19.8–26.0, 26.1–29.0, and >29.0 kg/m2) and GWG (<IOM, within IOM, >IOM recommendations). Because an upper limit of GWG for obese women has not been defined by the IOM, a cut-off value of 9.1 kg based on work by Lederman et al. (14) was used for the present analysis. The odds of unsuccessful initiation of BF were calculated for each of these 12 groups. Women with normal prepregnant BMI who gained within IOM recommendations served as the reference group. All models were adjusted for maternal age, parity, smoking, participation in WIC/PCAP, type of delivery, and whether the mother had attended college.
Analysis of the duration of BF. To address whether prepregnant BMI and GWG were associated with the duration of either EBF or ABF, proportional hazards analysis was used to calculate the hazard ratio of discontinuing BF over time in the 12 prepregnant BMI/GWG groups described above. Proportional hazards analysis provides the hazard ratio of discontinuing BF and takes into account not only whether a woman stopped breast-feeding her infant but also how long she breast-fed until her decision to stop. This approach uses data from all subjects, including those whose data were right-censored (1505 women for the analysis of EBF and 1252 women for the analysis ABF). Normal-weight women who gained weight within IOM recommendation served as the reference group. The analysis was conducted separately for the duration of EBF and ABF, and both regressions were adjusted for maternal age, parity, smoking, participation in WIC/PCAP, type of delivery, and whether the mother had attended college. For each outcome, the analysis included the interaction of prepregnant BMI and GWG categories.
To illustrate differences among the prepregnant BMI and GWG categories for the duration of EBF and ABF, we calculated the predicted values of these outcomes for a "reference woman" whose age was set to the mean of the sample (28.2 y) and who had characteristics that are uniformly associated with longer durations of BF (mulitparity, vaginal delivery, at least 16 y of education, not smoking before pregnancy, and not participating in WIC/PCAP).
RESULTS
Characteristics of the study subjects. Of the 2783 women who ever breast-fed their newborns, 13.1% were underweight, 54.7% were normal-weight, 12.7% were overweight, and 19.5% were obese before conception (Table 1). Compared with the other groups of women, the obese women gained less weight during pregnancy, were less educated, and breast-fed exclusively or to any extent for less time than normal-weight women (Table 1). At the other extreme, compared with all of the other groups, underweight women delivered lighter babies and a lower proportion of their infants were macrosomic (birth weight >4000 g) (Table 1).
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Duration of EBF. The predicted duration of EBF was 1 wk shorter for underweight and overweight women whose GWG was above the IOM recommendations and 3 wk shorter for obese women whose GWG was above the IOM recommendation (Fig. 2). In the hazard analysis, these differences were significant. In addition, obese women who gained below and within the IOM guidelines were also more likely to discontinue EBF than normal-weight women who gained within the guidelines (Table 6). There was no interaction between prepregnant BMI and GWG category on the duration of EBF. Both obesity (P < 0.001) and GWG above the IOM recommendations (P < 0.02) were significant as main effects in adjusted hazard regression models.
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Duration of BF to any extent. For obese women, the predicted duration of ABF was 17 wk shorter among those who gained within the IOM recommendation and 20 wk shorter among those who gained above the IOM recommendation than among women in the reference group (Fig. 2). In the hazard analysis, these differences were significant (Table 7). There was no interaction between prepregnant BMI and GWG category on the duration of ABF. As was the case for EBF, both obesity (P < 0.001) and GWG above the IOM recommendations (P < 0.03) were significant as main effects in adjusted hazard regression models.
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DISCUSSION
In this study, we identified a deleterious association of excessive weight gain during pregnancy with lactational performance that was in addition to that arising from maternal prepregnant BMI alone. Even normal-weight women who exceeded the IOM recommendations for GWG were at excess risk for failing to initiate BF successfully. From our earlier research in this same study population (2), we expected that obese women who gained within the IOM recommendations would have a greater risk of early termination of EBF or ABF, and this is indeed what we observed. What is new here is our finding that obese women who exceeded the IOM recommendations were at even greater risk for all of the poor BF outcomes that we examined. From our earlier research (2), we also expected to see that overweight women would be at risk of poor BF outcomes; this is what we observed, but it was significant only for the duration of EBF among those who exceeded the IOM recommendations for GWG. The trend was present among overweight women, but its failure to reach significance may have resulted from lack of adequate statistical power, which is discussed further below.
As has been the case in our other studies of the association between maternal fatness and lactational performance (2,3), we included in our study population only those women who actually attempted to breast-feed their newborn infants and focused on the first few days of BF. This is because, in our earlier studies in rats, we observed that excess maternal fatness was associated with early lactation failure (15). We showed that among those who attempted to breast-feed, those who did not successfully initiate BF (i.e., continue to breast-feed to any extent for at least 4 d) were not only heavier but also younger, less educated, poorer, and less likely to smoke than those who did successfully initiate BF. Gaining more than the IOM recommendations remained significantly associated with unsuccessful initiation of BF among both normal-weight and obese women after adjustment for possible confounding factors. This finding is the first indication that excessive GWG can be problematic for the success of BF among normal-weight women. The odds of unsuccessful initiation of BF were particularly high among the obese women whose GWG exceeded the IOM recommendation.
Gaining in excess of the IOM recommendations during pregnancy was also associated with a significantly shorter duration of EBF among underweight, overweight, and obese women, with the highest risk among obese women. This unique finding represents another new adverse outcome associated with excessive GWG. It is noteworthy that the effects of being too heavy before conception and gaining too much weight during pregnancy were additive, not synergistic, for the duration of EBF.
These findings are consistent with our observation (11) and that of Chapman and Perez-Escamilla (10) that being heavier than normal is associated with a delay in the onset of copious milk secretion (lactogenesis II), which normally occurs at d 3 postpartum. They are also consistent with our observation that heavier women produce less prolactin in response to suckling in wk 1 postpartum than normal-weight women (12). The especially high risk among obese women suggests that they may need additional support and assistance in the immediate postpartum period to be able to continue to breast-feed or to continue to do so exclusively.
There are few reports in the literature that provide the duration of ABF by categories of maternal prepregnant BMI. Beyond our earlier report from a subset of this white population (2), comparable data are available only from the report of Li et al. (8), which is from a large (n = 13,234) multiracial sample and included data from 7 states with 96% of the women living in low-income households. As expected, because of the differences in racial composition and income between that study and the present investigation, the duration of ABF in their sample was consistently several weeks shorter in each BMI category. Their report is the only other one to provide data on the duration of ABF by categories of GWG. Again, the duration of any BF in their sample was consistently several weeks shorter in each weight-gain category than in the present investigation.
Li et al. (8) found no interaction between prepregnant BMI and GWG on the duration of ABF. In adjusted analyses, underweight, overweight, and obese women breast-fed for shorter periods than normal-weight women, by 0.25, 0.5, 0, and 1.73 wk, respectively, but only the difference for obese women was significant. Those who gained less than and those who gained more than the IOM recommendations breast-fed for a shorter period than those who gained within the recommendations, by 0.75 and 0.84 wk, respectively. We, too, found no interaction between prepregnant BMI category and GWG category for the duration of ABF. To test for an interaction between gestational weight gain and maternal obesity (using 90% power and an 
level of 0.01), we calculated that 208 mother-infant dyads were needed in each of 12 categories of prepregnant BMI and GWG. Regrettably, we were not able to achieve this distribution of subjects, even having extended our data collection as far backward as possible while working with comparable medical records. Going further forward in time would not have helped because the trend was to have a progressively lower proportion of women in the least well-filled cells, namely, overweight or obese women who gained less than or within the IOM recommendation. This lack of adequate power appeared to be most important for the ABF outcome because associations of the same magnitude as for EBF were not significant for ABF, which had a higher variance.
There are 2 characteristics of this study that could pose potential threats to internal validity. First, the use of self-reported prepregnant weight could result in the miscalculation of prepregnant BMI and GWG and subsequent misclassification of BMI and GWG category. In another study in this population, Olson and Strawderman (16) compared measured weight in the first trimester and recalled prepregnant weight in the calculation of prepregnant BMI, which resulted in classification of women into the same BMI group 86% of the time. Nonetheless, 35% of women whose measured weight early in pregnancy put them in the obese category recalled a prepregnant weight in the normal-weight category. If this kind of misclassification were present in our data, it would have caused us to put obese women erroneously into the reference group (normal prepregnant BMI, GWG within the IOM recommendation) and, thus, to underestimate the association between high prepregnant BMI or excessive GWG and our outcomes. Second, although the reliability of information on infant feeding that is collected over a long period may be problematic, we did not find that to be the case in this study. This is because the questions about infant feeding that mothers of the infants completed at each well-baby visit were on a form that did not change over the 9-y period studied. Thus, we are confident that the reliability of the infant feeding information did not change over time.
Almost 50% of the women in the study population exceeded the recommendation for GWG for their prepregnant BMI category. These women had a higher proportion of cesarean deliveries and heavier infants, more than a quarter of whom were macrosomic. In another study in this same population, Olson and her co-workers (17) observed that GWG in excess of the IOM recommendations was associated with major weight gain (>4.55 kg or 10 lb) at 1 y postpartum. The risk of major weight gain associated with excess GWG was particularly high for women with high prepregnant BMI values and low income (i.e., those eligible for WIC/PCAP). Restricting GWG to the amount recommended by the IOM may help women to avoid not only the undesirable outcome of postpartum weight retention but also the problems with initiating and sustaining BF that were identified in this study. Although this is a laudable goal, it is clearly difficult to achieve (18). Therefore, more assistance with managing weight gain during pregnancy than is presently provided to most pregnant women will be needed to achieve this goal.
There are many mechanisms by which excess maternal fatness could negatively affect the success of BF. These include biological factors, such as a reduced prolactin response to suckling (12) and delayed onset of copious milk secretion (9–11) that may be associated with reduced milk production, as well as mechanical factors, such as difficulties with positioning the nursing infant and latching on, for which there is anecdotal evidence. In a recent study in mice, Flint and co-workers (19) observed abnormal development of the mammary gland during pregnancy as well as excess accumulation of lipid within the alveolar cells of the mammary gland at parturition in obese mice. These factors, as well as decreases in expression of mRNAs for key proteins and enzymes in the early postpartum period help to explain the impaired lactogenesis that was observed in these obese mice. We were unable to show that psychosocial or sociodemographic characteristics of the mother were responsible for this association (11), but a contribution from these factors cannot yet be excluded. To our knowledge, no attempt has yet been made to disentangle the separate contributions of excess fatness acquired before pregnancy from that acquired during pregnancy to these biological and mechanical factors related to reduced milk production.
Li et al. (8) observed that gaining less than the IOM recommendations may also not be optimal for the duration of ABF. Our data suggest that this is likely to be the case for initiation of BF and the duration of EBF as well. Unfortunately, our analyses lacked adequate statistical power in a number of the BMI/GWG cells, but the OR were often as high as for gaining more than the IOM recommendations. Taken together, these observations suggest that further study of the association of inadequate GWG with lactational performance is warranted.
In this investigation we learned that in all categories of prepregnant BMI, excessive GWG was associated with one or more measures of failure to initiate and/or sustain BF. These associations were independent of and in addition to those observed previously for the prepregnant BMI. Although we do not yet know what it is about being too heavy before conception or gaining excessively during gestation that is associated with failure to initiate and/or sustain BF, women can nonetheless be counseled to conceive at a healthy weight and to gain the recommended amount of weight during pregnancy. These are difficult tasks, but ones that are essential if we are to be able as a nation to meet our goals for BF and, thereby, for the promotion of maternal and infant health.
ACKNOWLEDGMENTS
We thank Jane O'Brien of the medical records staff at Mary Imogene Bassett Hospital for her patience and assistance with the chart review. We also thank Dr. Edward A. Frongillo, Jr. and Cara Olson for their helpful statistical advice and Dr. Katarzyna Kordas for additional analytic assistance.
FOOTNOTES
1 Presented in part at the 10th International Conference of the International Society for Research in Human Milk and Lactation held September 15-19, 2000, Tucson, AZ (Rasmussen KM, Hilson JA, Kjolhede CL. Obesity as a risk factor for failure to initiate and sustain lactation. Adv Exp Med Biol. 2002;503:217-22). 
2 Supported by National Institutes of Health (Training in Maternal and Child Nutrition, T32 HD07331) and U.S. Department of Agriculture-Hatch (Obesity and the Initiation of Lactation, NYC399402).
4 Abbreviations used: ABF, any BF; BF, breast-feeding; EBF, exclusive BF; GWG, gestational weight gain; IOM, Institute of Medicine; OR, odds ratio; PCAP, Prenatal Care Assistance Program; WIC, Special Supplemental Nutrition Program for Women, Infants and Children.
Manuscript received 23 August 2005. Initial review completed 22 September 2005. Revision accepted 29 September 2005.
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