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American Journal of Clinical Nutrition, Vol. 87, No. 6, 1750-1759, June 2008
© 2008 American Society for Nutrition

ORIGINAL RESEARCH COMMUNICATION

Combined associations of prepregnancy body mass index and gestational weight gain with the outcome of pregnancy1,2,3

Ellen A Nohr1, Michael Vaeth1, Jennifer L Baker1, Thorkild IA Sørensen1, Jorn Olsen1 and Kathleen M Rasmussen1

1 From the Departments of Epidemiology (EAN) and Biostatistics (MV), Institute of Public Health, University of Aarhus, Aarhus, Denmark; the Institute of Preventive Medicine, Copenhagen University Hospital, Centre of Health and Society, Copenhagen, Denmark (JLB and TIAS); the Department of Epidemiology, School of Public Health, UCLA, Los Angeles, CA (JO); and the Division of Nutritional Sciences, Cornell University, Ithaca, NY (KMR)

2 Supported by a major grant from The Danish National Research Foundation (which established the Danish Epidemiology Science Centre, which initiated and created the Danish National Birth Cohort) and by the Pharmacy Foundation, the Egmont Foundation, the March of Dimes Birth Defects Foundation, and the Augustinus Foundation.

3 Reprints not available. Address correspondence to E Aagaard Nohr, Department of Epidemiology, Institute of Public Health, Vennelyst Boulevard 6, Building 260, University of Aarhus, 8000 Aarhus C, Denmark. E-mail: ean{at}soci.au.dk


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: Although both maternal prepregnancy body mass index (BMI) and gestational weight gain (GWG) may affect birth weight, their separate and joint associations with complications of pregnancy and delivery and with postpartum weight retention are unclear.

Objectives: We aimed to investigate the combined associations of prepregnancy BMI and GWG with pregnancy outcomes and to evaluate the trade-offs between mother and infant for different weight gains.

Design: Data for 60892 term pregnancies in the Danish National Birth Cohort were linked to birth and hospital discharge registers. Self-reported total GWG was categorized as low (<10 kg), medium (10–15 kg), high (16–19 kg), or very high (≥20 kg). Adjusted associations of prepregnancy BMI and GWG with outcomes of interest were estimated by logistic regression analyses.

Results: High and very high GWG added to the associations of high prepregnancy BMI with cesarean delivery and were strongly associated with high postpartum weight retention. Moreover, greater weight gains and high maternal BMI decreased the risk of growth restriction and increased the risk of the infant's being born large-for-gestational-age or with a low Apgar score. Generally, low GWG was advantageous for the mother, but it increased the risk of having a small baby, particularly for underweight women.

Conclusions: Heavier women may benefit from avoiding high and very high GWG, which brings only a slight increase in the risk of growth restriction for the infant. High weight gain in underweight women does not appear to have deleterious consequences for them or their infants, but they may want to avoid low GWG to prevent having a small baby.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
It has been explicitly recognized since at least 1990, when the Institute of Medicine (IOM) last revised recommendations for weight gain during pregnancy (1), that both prepregnancy body mass index (BMI) and gestational weight gain (GWG) are associated with the outcome of pregnancy. A large body of data links a high prepregnancy BMI with a number of fetal and maternal complications, including fetal death, preeclampsia, gestational diabetes, macrosomia, and complicated deliveries (2–5). GWG has also been thoroughly studied as a predictor of adverse pregnancy outcomes, mainly because of the belief that it is potentially modifiable after conception. Low gain is associated with birth of a small-for-gestational age (SGA) infant (4, 6, 7) and preterm birth (8, 9), whereas high gain is associated with greater risks of macrosomia (6, 7, 10), cesarean section (11, 12), and excess postpartum weight retention (13–15).

The nature of the combined effects of prepregnancy BMI and GWG on maternal and neonatal aspects of the outcome of pregnancy has been considered in more detail (16–20). In particular, it has been suggested that obese women may benefit from low GWG (16, 17, 19).

Understanding these associations is an important issue for several reasons. Women in the Western world are heavier than ever, and being pregnant contributes further to obesity (21, 22). Moreover, US data show that the proportion of women with excessive GWG has increased during the past 15 y (23, 24). Maternal and neonatal complications associated with BMI and GWG are of public health importance because they add to the disease burden in women and children and increase medical costs. However, understanding these associations is also complex, because both BMI and GWG are closely linked to lifestyle factors, diseases, and genetic traits that are also correlated with the outcome of pregnancy. In addition, pregnancy outcomes (eg, birth weight) may be in the causal pathway between GWG and other pregnancy outcomes (eg, cesarean delivery), which also complicates the interpretation of these relations.

When BMI-specific ranges for "optimal" weight gain were introduced, the IOM considered only infant—but not maternal—health as a consequence of GWG, in part because adequate data on the maternal health consequences of variations in GWG were not available at the time. Data are now available that permit us to examine the association of GWG with both maternal and infant health outcomes. Here we use data, drawn from a large contemporary sample of white women, to investigate the contributions of GWG in addition to those of prepregnancy BMI to the course and outcome of pregnancy and to postpartum weight retention. In addition, we examine whether the GWG values associated with good outcomes for the newborn are also associated with good outcomes for the mother.


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects
The Danish National Birth Cohort (DNBC) is a nationwide study of 100419 pregnancies among 92274 women recruited in 1996–2002. Detailed descriptions of the study methods and the recruitment were previously published (25–27). Briefly, data were collected during 2 telephone interviews at {approx}16 and 30 wk of gestation and during postnatal telephone interviews when the child was {approx}6 mo and 18 mo old.

For this study, we initially considered 64592 pregnancies that resulted in liveborn, full-term singletons (≥37 wk of gestation) and in which the mothers had participated in the first pregnancy interview (92.4% of the entire cohort) and the first postpartum interview (69.6% of the entire cohort). From this group, we excluded women with type 1 diabetes (n = 129) and those <18 y old (n = 71). We also excluded women with missing information about important study variables, ie, prepregnancy BMI (n = 1028), GWG (n = 553), and weight retention 6 mo after birth (n = 1919). These exclusions resulted in a final study population of 60892 pregnancies among 57700 women.

All participants provided written informed consent. The study was approved by all of the scientific ethics committees in Denmark and by the Danish Data Protection Board.

Exposure variables
The main exposures were prepregnancy BMI and GWG. On the basis of self-reported information on weight and height from the first pregnancy interview, prepregnancy BMI (in kg/m2) was categorized as underweight (BMI < 18.5), normal-weight (18.5 ≤ BMI < 25), overweight (25 ≤ BM I < 30), or obese (BMI ≥ 30) (28). GWG was based on information from the telephone interview 6 mo after birth, when the woman was asked "How much (in kg) was your total gain in pregnancy?" GWG was categorized as low (<10 kg), medium (10–15 kg), high (16–19 kg), or very high (≥20 kg), which corresponded to the 12th, 58th, and 79th percentiles of the distribution, respectively. These cutoffs were chosen to allow precise estimation of risks related to low weight gain and to different degrees of high weight gain relative to the reference category (ie, the medium category) that has been associated with minimum mortality for the infant in other populations (1).

From the first pregnancy interview, we also obtained information about the mother's age at conception; parity; lifestyle habits in the first part of pregnancy, including smoking, alcohol intake, and physical exercise; and social status defined by education and occupation. We used information about duration of breastfeeding taken from the first postpartum interview. Women who reported an exact span of breastfeeding duration (in wk) or who were still breastfeeding were included in 1 of the following 3 categories: 0–13wk, 14–21 wk, or >22 wk. The lowest category also included women who never breastfed (n = 718) or did not provide information about breastfeeding (n = 13).

Maternal outcomes
Pregnancy outcomes during late pregnancy were identified through linkage to the National Hospital Discharge Register (NHDR) by use of the personal identification number. Pregnancies complicated by preeclampsia or eclampsia were identified by the diagnosis of the International Classification of Diseases (ICD)–10 codes O14 or O15 in the NHDR. Codes I10 through I15 and codes O10, O11, and O13 were used to identify chronic or gestational hypertension; for diabetes, we used the codes E10 and O24. We divided diabetes into pregestational and gestational diabetes and, because we suspected some underreporting of the latter disease, we added self-reported information from the pregnancy interviews in which the woman was asked if she was diagnosed with gestational diabetes during pregnancy. This increased the proportion with this disease from 0.9% to 1.2%.

Birth complications were identified in the NHDR. They included instrumental deliveries, which in nearly all cases covered vacuum extraction, and planned and emergency cesarean sections.

Postpartum weight retention was calculated as the difference between the woman's prepregnancy weight and her weight 6 mo after delivery, as reported in the first postpartum interview. Postpartum weight retention was summarized by 2 variables, defined as postpartum weight loss (a loss of ≥2 kg from prepregnancy weight) and postpartum weight retention (a gain of ≥5 kg over prepregnancy weight). These cutoffs were driven by the distribution in the data, and they corresponded to the 22nd and 78th percentile, respectively. We also calculated postpartum weight retention at 18 mo for those women in the study population who participated in the second postpartum interview, who had not given birth again, and who were not pregnant again. Here we implemented the same cutoffs as for 6 mo.

Neonatal outcomes
Neonatal outcomes were identified in the National Birth Register; they included birth weight, length at birth, gestational age as recorded at birth, and Apgar score after 5 min. Birth weight was standardized for gestational age by calculating a z score according to the method suggested by Marsal et al (29), which is based on fetal weights derived from serial ultrasound measurements in a Scandinavian population. A small-for-gestational-age (SGA) infant was defined as an infant having a standardized birth weight <10th percentile, whereas a large-for-gestational-age (LGA) infant was defined as an infant having a standardized birth weight >90th percentile. To describe the fatness of the infant, we also calculated the ponderal index (PI) of the newborn (birth weight in g divided by the birth length in cm3) (30). We defined a low PI as a value <10th percentile and a high PI as a value >90th percentile. A low Apgar score was defined as a value of <8 after 5 min.

Statistical analysis
In multiple logistic regression models, categories of GWG and prepregnancy BMI were mutually adjusted to estimate the independent associations with a number of pregnancy outcomes and birth complications. Medium GWG and normal-weight BMI were used as reference groups. In these models, we also adjusted for a number of maternal characteristics and lifestyle factors that are known to correlate with some of the endpoints under study. To account for the fact that length of gestation may influence both GWG and the risk of pregnancy complications, we also adjusted for gestational age at birth as a continuous variable. This model is referred to as the basic model.

First, we estimated adjusted odds ratios (ORs) for pregnancy outcomes during late pregnancy including preeclampsia, other hypertensive disorders, and gestational diabetes. Next, we estimated adjusted ORs for a number of birth complications and neonatal outcomes. In these analyses, women with preeclampsia (n = 1118) or gestational diabetes (n = 669) were excluded because GWG in these groups may either be a part of a disease or be controlled during pregnancy. In addition, these diseases may be on the causal pathway between GWG and birth complications.

In the analyses of instrumental deliveries, cesarean delivery, and low Apgar score, adjustment for birth weight, in 5 categories (cutoffs of 3.0, 3.5, 4.0, and 4.5 kg), was added to the basic model. Inasmuch as birth weight may be on the causal pathway between prepregnancy BMI, GWG, and these labor complications, we repeated the analyses without this additional adjustment.

The ORs for postpartum weight retention or loss 6 mo after delivery also were based on pregnancies without preeclampsia and gestational diabetes. Here, we added to the basic model the exact time of the interview (wk after birth), and we also adjusted for breastfeeding. Inasmuch as breastfeeding may be on the causal pathway between prepregnancy BMI, GWG, and postpartum weight retention, we repeated these analyses without adjustment for breastfeeding.

To illustrate the predictions of the fitted models, we computed the estimated risks for each category in the cross-classification of prepregnancy BMI by GWG. These risks were computed from the estimated ORs for a woman with a given set of confounder categories. In all adjusted models, Wald's test with 6 df was used to assess the hypothesis that there was no effect modification by BMI group of the association between GWG and pregnancy outcomes. We used a significance level of 0.05 in all statistical tests, and ORs are presented with 95% CIs. A correction for within-cluster correlation (robust standard errors) was applied in all models because 3192 women contributed >1 birth to the study.

For each category of GWG, we cross-classified the study population by prepregnancy BMI category and the BMI category 6 mo after delivery to illustrate the change in BMI category after the birth. To better highlight the differences among the obese women, the obese category was here divided into 2 subcategories—obese class 1 (30 ≤ BMI < 35) and obese class 2 and 3 (BMI ≥ 35). We used STATA software (version 9.1 Special Edition; Stata Corp, College Station, TX) for all statistical analyses.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
The mean GWG in this population was 15.1 kg, which was fairly stable across the prepregnancy BMI groups except obese women, whose mean gain was only 10.5 kg (Table 1Go). The variation in weight gain was high, and it also increased across BMI groups. Nearly half of the underweight and normal-weight women gained 10–15 kg, and a similar proportion exceeded 15 kg, which left only 8% of these groups with low GWG (<10 kg). Low gain was more common among both overweight and obese women than among underweight and normal-weight women, and >40% of the obese women gained <10 kg.


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  		TABLE 1 Gestational weight gain (GWG) and postpartum weight retention at 6 mo (PPWR) across prepregnancy BMI and weight gain categories1

 
Underweight, overweight, and obese women were slightly younger and more likely to be multiparous or of short stature than were normal-weight women (Table 2Go). During pregnancy, they smoked more, drank less alcohol, exercised less, and had lower socioeconomic status (SES) than did normal-weight women. Overweight and obese women breastfed for a shorter period than did normal-weight women. Women with low GWG were more often older than 34 y, multiparous, or of short stature than were women with higher GWG. Heavy smoking and consumption of alcohol during pregnancy were more frequent among low weight gainers than among women with medium weight gain, as were a lower SES and a shorter period of breastfeeding. Typically, women with higher weight gains were tall, <30 y old, and primiparous. Very high weight gain was more frequent among women of lower SES, light smokers, women who did not consume alcohol or exercise during pregnancy, and women with a shorter period of breastfeeding.


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  		TABLE 2 Maternal characteristics across prepregnancy BMI and gestational weight gain categories1

 
Pregnancy complications
During late pregnancy, hypertension, gestational diabetes, and preeclampsia were all considerably more common in overweight and obese women than in normal-weight women (Table 3Go). The additional risk contributed by GWG was modest, except that preeclampsia was strongly associated with higher weight gains and gestational diabetes was strongly associated with low weight gains.


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  		TABLE 3 Maternal outcomes according to prepregnancy BMI and gestational weight gain categories1

 
Birth complications
The risk of cesarean delivery, especially on an emergency basis, increased with increasing BMI (Table 3Go). GWG added only modestly to these associations, but there was a trend of greater risk with higher gains. The same pattern, although less pronounced, was observed for instrumental deliveries; however, underweight women also faced a greater risk. Removal of the adjustment for birth weight resulted in a marginal increase of the associations between high prepregnancy BMI or high GWG and emergency cesarean delivery (data not shown).

Neonatal outcomes
The risk of giving birth to an SGA infant decreased with increasing prepregnancy BMI (Table 4Go). The reverse pattern was seen for the risk of giving birth to an LGA infant. Findings related to GWG showed similar trends of equal strength: with increasing GWG, the risk of delivering an SGA infant decreased and the risk of delivering an LGA infant increased. Estimates for low and high PIs showed the same picture as those for SGA and LGA, albeit less pronounced (data not shown). The risk of a low Apgar score increased with increasing BMI and GWG of the mother.


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  		TABLE 4 Neonatal outcomes according to prepregnancy BMI and gestational weight gain categories1

 
Postpartum weight retention
Mean weight retention 6 mo after birth differed among the prepregnancy BMI groups (Table 1Go). Underweight women retained on average 2.3 kg, and obese women lost 1.7 kg, but variation was high among overweight and obese women. As a result, the crude risks for postpartum weight retention of ≥5 kg did not vary greatly among the BMI groups (Table 3Go), and the adjusted ORs for postpartum weight retention showed only modest associations with prepregnancy BMI. In contrast, the crude risks differed considerably across categories of GWG: nearly half of the women with GWG of ≥20 kg retained ≥5 kg, but only 6% of women with low GWG did so. The varying risk resulted in strong associations between GWG and postpartum weight retention (Table 3Go). Thus, compared with women who gained 10–15 kg, the adjusted odds for postpartum weight retention were 6 times as high in women with gains of ≥20 kg. Removal of the adjustment for breastfeeding did not change the results.

Prepregnancy BMI and GWG were equally important to the risk of losing ≥2 kg relative to one's prepregnancy weight (Table 3Go). Women with low weight gains and obese women had a 50% chance of being ≥2 kg lighter after pregnancy than before pregnancy. It may be clinically relevant that, among overweight and obese women who gained <10 kg during pregnancy, this chance was as high as 54% and 66%, respectively (results not shown).

Data on weight retention 18 mo after birth was available for 39776 women. For this outcome, the associations with prepregnancy BMI were similar to those 6 mo after delivery, whereas those with GWG were somewhat attenuated (data not shown).

Interactions
For most of the outcomes that we examined, there was no statistical interaction between the associations related to prepregnancy BMI and GWG. However, for SGA, LGA, and high postpartum weight retention, prepregnancy BMI modified the association of these outcomes with GWG. We found these interactions to be of minor clinical significance except for the risk of SGA in underweight women with low gain. (See Figure S1 under "Supplemental data" in the current online issue.) In the basic model in which BMI and GWG were independent risk factors, the OR for this group was only 3.4 (1.8 x 1.9) compared with 5.5 if an interaction term were added to the model. Thus, an underweight woman with a low GWG faced a risk of giving birth to a small baby >5 times that faced by an underweight woman with a normal GWG.

Possible trade-offs between mother and infant
To illustrate the possible trade-offs between mother and infant for various GWG values, we present BMI-specific effects of GWG on SGA, LGA, emergency cesarean delivery, and postpartum weight retention on an absolute risk scale (Figure 1Go). The adjusted risk of SGA in underweight women was particularly high in the low GWG group. Thus, an underweight, primiparous woman with low GWG had a 47% risk of giving birth to a growth-restricted infant. This risk was reduced to 10% if she had very high GWG, which corresponded to a risk reduction of 37%. The similar risk reductions for normal-weight, overweight, and obese women were only 16%, 11%, and 5%, respectively. The opposite pattern was seen for LGA and cesarean delivery during delivery. Here, an underweight woman increased her absolute risk by only 3–4% by moving from low to very high GWG. For the other BMI groups, the similar increase in absolute risk was 7–8% for normal-weight, 8–10% for overweight, and 10–12% for obese women. The absolute risk of postpartum weight retention in relation to GWG did not differ significantly across BMI groups.


Figure 1
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  		FIGURE 1. Adjusted absolute risks for small-for-gestational-age (SGA, •), large-for-gestational-age (LGA, {circ}), emergency cesarean section (CS; {blacktriangleup}), and postpartum weight retention of ≥5 kg (PPWR, {triangleup}) according to prepregnancy BMI (in kg/m2) and gestational weight gain categories of the World Health Organization. BMI categories are underweight (<18.5), normal-weight (18.5–24.9), overweight (25–29.9), and obese (≥30); gestational weight gain categories are low (<10 kg), medium (10–15 kg), high (16–19 kg), and very high (≥20 kg). Points represent the risks of a primparous woman 25–29 y old and 1.60–1.69 m tall who is not a smoker or alcohol consumer, who has high social status, and who does no exercise, at 280 d of gestation. For PPWR, she is breastfeeding <14 wk. Of 58342 (SGA and LGA), 58693 (PPWR, and 54810 (emergency CS) women, 4% were underweight, 69% were normal-weight, 19% were overweight, and 7% were obese.

 
Finally, we calculated the proportion of women who had changed from one prepregnancy BMI group to another by 6 mo after delivery according to their category of GWG (Table 5Go). Among women who gained <10 kg during pregnancy, 24% of the initially underweight women moved to the normal-weight category. Only 5% of the initially normal-weight became underweight—and an equal proportion became overweight even with a GWG that low. When their GWG was <10 kg, {approx}25% of those women who were initially overweight and {approx}33% of those who were obese or extremely obese moved down a BMI category. With a GWG of 16–19 kg, 12% of normal-weight women became overweight, 12% of overweight women became obese, and 14% of obese women became extremely obese. Among women who gained ≥20 kg, <1% of the initially underweight women moved to the overweight group, but {approx}25% of the women who initially belonged to the normal-weight, overweight, or obese class 1 group moved up one BMI category.


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  		TABLE 5 Migration in BMI category at 6 mo after delivery according to prepregnancy BMI1

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In this study, we identified infant size at birth, emergency cesarean delivery, and postpartum weight retention as important outcomes in which the trade-offs between mother and infant may differ according to prepregnancy BMI. Our findings corroborate the idea contained in the IOM recommendations for GWG (1)—ie, that underweight women may gain weight rather freely without deleterious consequences for themselves or their infants. Our findings also show that heavier women may be wise to avoid a high or very high GWG. We found that prepregnancy BMI was by far the strongest predictor of the outcomes under study. The contribution of GWG was modest except for infant size at birth and postpartum weight retention.

The DNBC provided information about postpartum weight retention, which allowed us to consider maternal and neonatal outcomes within the same population to a greater extent than was previously done. In addition, we had more detailed information about potential confounding factors than that available in most other large studies of GWG (7, 16–19). Nonetheless, these are observational data used to study quite complex exposures, and some uncontrolled confounding is likely to remain, even in our adjusted analyses. The associations studied are those that remained after interventions provided by the Danish medical system. Moreover, the study population was mostly white. We have no way of knowing whether the results apply to other racial-ethnic groups.

As has been the case for other studies (6, 21, 31), we relied on self-reported information about prepregnancy BMI and GWG. In a subsample of the cohort, we previously validated self-reported prepregnancy weight and self-reported GWG relative to the same measures observed in antenatal care (32). The BMI categories derived from the self-reported and observed weights agreed for 91% of the women. As expected for a measure of difference, the agreement rate was lower—only 64% for GWG. If these findings were extended to the population of the present study, risk estimates related to prepregnancy obesity and GWG would be underestimated by {approx}5% and 15%, respectively.

We used total GWG as our measure of exposure because we wanted to describe associations with pregnancy outcomes in full-term pregnancies. We categorized total gain with cutoffs that allowed us to estimate risks related to low gain and to different degrees of high weight gain with sufficient precision. This allowed us to show that, for most pregnancy outcomes, higher gains were progressively more problematic.

We found, as have others (16, 18, 33), that GWG was associated with complications during late pregnancy, such as preeclampsia and gestational diabetes. However, any causal interpretation of the association between total weight gain and these complications is limited. For preeclampsia, high total gain most likely reflects pathologic fluid retention as part of the disease, and it may be present before other, more serious symptoms. For gestational diabetes, reverse causation may explain the association—that is, screening is carried out more often among obese than lower-weight women, and, after diagnosis, the obese women are often prescribed a diet that will restrict their total gain. Measuring weight gain only until the time of diagnosis is a way to solve these problems, but we lacked the data to do this. We only studied full-term pregnancies; thus, if the most severe cases presented as preterm deliveries, they were excluded from our analyses.

The strong association between increasing GWG and increasing birth weight was similar to that reported by numerous other studies (4, 6, 7, 10, 16, 18, 19). As was the case in the present findings, high GWG has also been associated with cesarean delivery (11, 16, 20), especially during labor (12, 34); with instrumental deliveries (16, 33); and with low Apgar scores (7). It has been suggested that these associations are mediated by birth weight (7, 11, 20). In the present data, this was the case only (and then to a limited degree) for the association between GWG and emergency cesarean delivery.

High GWG has also been associated with higher postpartum weight retention in some much smaller studies (13–15). At the population level, childbearing is associated with a modest increase in weight that most likely reflects a combination of retention of GWG and weight change due to lifestyle changes related to child-rearing (35). High GWG has also been associated with weight retention 15 y later (21). The fat that is gained during pregnancy is preferentially deposited on the trunk and thighs (36), but the fat that is retained with increasing parity is also deposited in the intraabdominal compartment (37). Thus, to the extent that weight retained after delivery is fat, it represents a concern for women's later health. Moderation of GWG is one of the few options open to women for limiting postpartum weight retention. It is therefore an important finding that 54% of overweight and 66% of obese women in the DNBC with low GWG were ≥2 kg below their prepregnancy weight at 6 mo after delivery.

The observed effect of GWG tended to be similar across BMI groups, and statistical interactions in a multiplicative model between the influences of prepregnancy BMI and GWG were limited to expressions of infant size at birth and postpartum weight retention. However, these outcomes were by definition frequent in this large data set, and thus statistical interaction was not unexpected. Except for the strong association between low GWG and fetal growth restriction in underweight women, these interactions seemed to be of modest importance. This suggests that BMI-specific recommendations for GWG should be based on the different background risks of pregnancy outcomes within these groups.

When we evaluated trade-offs between the mother and her infant with respect to GWG on an absolute-risk scale, our data highlighted that infants of underweight primiparae were especially susceptible to fetal growth restriction and could possibly benefit from high GWG without facing a substantial risk of macrosomia. For underweight women, the increase in risk of emergency cesarean sections with higher gains was limited, and postpartum weight retention may represent an opportunity for them to normalize their body weight. Thus, for these outcomes, high GWG was probably not disadvantageous for either underweight women or their infants. These findings do not agree with a recent study (17) that suggests 4–10 kg as the optimal GWG for underweight women.

For the other BMI groups, the point at which disadvantages of high gains outweighed their advantages moved toward lower gains with increasing BMI. Our data suggest that, for normal-weight women, this point was in the middle of the high-gain category; for overweight women, it was in the middle of the medium category; and for obese women, it was in the low category. If one adds postpartum weight retention and a range of other pregnancy outcomes to this synthesis, the argument of recommending lower GWG in those with higher BMIs is strengthened. However, this exercise is not trivial and should also consider a woman's individual risk profile and the long-term health consequences for her and her child.

Lifestyle interventions that are aimed at restricting weight gain in obese women during pregnancy are seen as a tool to fight obesity in antenatal care, but >40% of the obese women in this study had weight gains of <10 kg without intervention. It may be a better investment to pay more attention to women at higher risk of becoming overweight, obese, or even more obese as a result ofpregnancy. The need remains, however, for a randomized trial designed to help women enter pregnancy at a healthy BMI, either by losing weight before their first conception or between pregnancies. More important, such a trial could also provide evidence of a causal relation between obesity and pregnancy complications.


   ACKNOWLEDGMENTS
 
The authors' responsibilities were as follows—EAN and KMR: conceived of the study; EAN: prepared the data set for analysis; EAN and MV: conducted the data analysis; EAN and KMR: prepared the manuscript; MV, JLB, TIAS, and JO: critical revision of the manuscript; and JO and TIAS: led the overall data acquisition of the Danish National Birth Cohort. None of the authors had a personal or financial conflict of interest.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication October 26, 2007. Accepted for publication February 10, 2008.





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