Birth weight is an important predictor for survival and health. Infants with a low birth weight (less than 2,500 g or less than 3,000 g) have increased perinatal morbidity and mortality as well as an increased risk of adulthood hypertension and type 2 diabetes.1–5 For many years the focus has been on increasing birth weight, which is positively correlated with maternal weight gain during pregnancy. However, although a high maternal weight gain will tend to decrease the incidence of low birth weight, an excessive weight gain may entail fetal as well as maternal complications, such as pregnancy and delivery complications and obesity later in life.6–8 Several studies have shown that delivering an infant with a birth weight more than 4,000 g increases the risk of severe intrapartum complications, such as shoulder dystocia and both infant and mother injuries.9 Having a birth weight more than 4,000 g also entails an increased risk of certain types of diseases in adulthood, eg, breast cancer in women and an increased risk of type 2 diabetes mellitus.3,4,10
Prepregnancy body mass index (BMI) is an important predictor of birth weight. It is therefore important to combine maternal weight gain and prepregnancy BMI in analyses of the association between weight gain and infant birth weight. In the 1990s, the American Institute of Medicine (IOM) introduced weight gain recommendations for pregnant women with different recommendations for underweight (BMI less than 19.8 kg/m2), normal weight (19.8–26.0 kg/m2), overweight (26.1–29.0 kg/m2), and obese women (BMI more than 29.0 kg/m2).11 Similar weight gain recommendations do not exist in Scandinavia as yet.
It is imperative to investigate the relation between maternal weight gain and birth weight further, because maternal weight gain may be subject to regulation through pregnancy counseling. The aim of this study was to describe the association between maternal weight gain and birth weight in four prepregnancy BMI groups (underweight, normal weight, overweight, and obese women), with emphasis on the use of IOM recommendations in Denmark. Furthermore, the purpose was to examine differences of birth weight less than 3,000 g and 4,000 g or more in term infants in women with prenatal weight gain above, within, and below the recommended range of weight gain for each of the four prepregnancy BMI groups.
MATERIALS AND METHODS
We used data from the “Smoke-free Newborn Study”12 in this substudy on the association between maternal weight gain and birth weight. The “Smoke-free Newborn Study” was conducted at a large university hospital in Copenhagen and was approved by the Scientific Ethics Committee of the cities of Copenhagen and Frederiksberg (No. KF-02–084/95). Informed consent was obtained from all participants. Participants completed two questionnaires, one after their visit with the midwife between 12 and 18 weeks of gestation and another at 37 weeks of gestation. A total of 4,000 women (aged 18 years or older, fluent in Danish, without alcohol or drug abuse) were invited to participate, and 3,206 women accepted this invitation. Data were collected from November 1996 until October 1998. The questionnaires provided information about sociodemographic history (age, years in school, marital status, prepregnancy weight and height), obstetric history (parity, weight at 37 weeks of gestation, preeclampsia), and life-style factors (caffeine intake, alcohol intake, and cigarette consumption). Information about gestational age and birth weight was provided from the obstetric journal.
In this substudy, we included data from women who had answered a questionnaire at both 12–18 and 37 weeks of gestation (N=2,387), but excluded women with multiple gestation and women who delivered before 37 weeks of gestation (N=113). Information on the infants’ birth weight was not available in 26 cases. The total number of women included in the final study population was 2,248.
Information on preeclampsia was obtained from the department's obstetric database and was added to the project database. In Denmark, preeclampsia is defined as hypertension (systolic blood pressure greater than 140 mm Hg, diastolic blood pressure greater than 90 mm Hg or an increase of 20 mm Hg in the diastolic blood pressure during pregnancy) and significant proteinuria (protein more than 0.3 g/d).
We categorized the following variables: maternal age, parity, gestational age at delivery, marital status, years in school, alcohol intake, smoking status, caffeine intake, and preeclampsia.
We stratified all women in the study population according to BMI into four groups: BMI less than 19.8 kg/m2 (underweight), BMI 19.8–26.0 kg/m2 (normal weight), BMI 26.1–29.0 kg/m2 (overweight), and BMI greater than 29.0 kg/m2 (obese). For the logistic regression analyses, we categorized maternal weight gain according to IOM recommendations (less than recommendation, within recommendation, or above recommendation) in the four BMI groups.
Our outcome variable was birth weight, which was categorized as birth weight less than 3,000 g and birth weight of 4,000 g or more. These two birth weight limits represented the lower 10th percentile and the upper 20th percentile in the population.
We used the SPSS 12.0 software (SPSS Inc., Chicago, IL) for all statistical analyses. The analyses included linear regression analysis, χ2 testing, and logistic regression analysis of the associations between maternal weight gain and infant birth weight less than 3,000 g and 4,000 g or more in underweight, normal weight, overweight, and obese women. If one of the potential confounders changed the estimated association between weight gain (less than recommendation, within recommendation, or above recommendation) and birth weight categories by at least 10%, it remained in the final model.13 Results of the logistic regressions were expressed as odds ratios (ORs) including the corresponding 95% confidence intervals (CI).
Table 1 shows the distribution of demographic, obstetric, and lifestyle variables according to BMI group in the 2,248 pregnant women, and Figure 1 shows the distribution of weight gain (5-kg groups) in the four BMI groups. The mean weight gain was 14.5 kg in both underweight and normal weight women and 12.7 kg in overweight women, but only 8.4 kg in obese women (P<.001). The mean birth weight was 3,499 g in underweight, 3,619 g in normal weight, 3,660 g in overweight, and 3,770 g in obese women (P<.001).
Figure 2 shows an inverse relationship between maternal weight gain and the proportion of infants with a birth weight less than 3,000 g in underweight women (R2=0.5523), whereas the proportion of infants with a birth weight of 4,000 g or more augments with an increasing weight gain (R2=0.4424). In normal weight women the rate of birth weight less than 3,000 g levels out at a weight gain more than 15 kg (R2=0.5404, log linear regression), whereas the rate of birth weight of 4,000 g or more continues to increase dramatically with an increasing weight gain (R2=0.8024). The association between weight gain and infant birth weight was less apparent in the overweight and obese women, where all R2 values in the linear regressions were less than 17%. No obese women with a weight gain more than 15 kg delivered an infant with a birth weight less than 3,000 g.
Table 2 shows the results from the logistic regression analyses using the IOM recommendation for each BMI group as the reference value. The OR for birth weight less than 3,000 g was significantly reduced in underweight women who gained more than the recommended 18 kg, but the seeming concurrent increase in birth weight of 4,000 g or more was not significant. In normal weight women, there was an increased risk of birth weight less than 3,000 g in the group who gained less than 11.5 kg, and an unchanged risk in women who gained more than 16 kg. However, the risk of birth weight of 4,000 g or more was decreased in normal weight women who gained less than the recommended 11.5 kg and increased with an OR of 1.9 (CI 1.5–2.5) in women who gained more than the recommended 16 kg. The ORs for overweight women were not significant, but were comparable with the ORs for normal weight women. We did not find an association between weight gain and birth weight less than 3,000 g or of 4,000 g or more in obese women.
Table 2 shows that a total of 130 of 385 (34%) underweight women in this population gained less than 12.5 kg. Thirty-nine percent (61 of 155) of obese women gained less than 7 kg (nearly 10% gained less than 1 kg, Fig. 1). Nearly 40% (607 out of 1,531) of normal weight women gained more than the recommended 16 kg, and approximately 11% gained more than 20 kg (Fig. 1).
Due to the rather small sample size for analyses of birth weight less than 2,500 g and 4,500 g or more, these analyses had limited power for underweight, overweight, and obese women. They did, however, show similar trends. No underweight women with a weight gain above the recommended weight gain had an infant with a birth weight less than 2,500 g, but the incidence of birth weight of 4,500 g or more rose to 5% compared with 2.8% in the women who gained weight according to recommendations (not significant). In normal weight women, gaining less than recommended seemed to double the risk of having an infant with a birth weight less than 2,500 g (P=.09), and there was a significant association between birth weight of 4,500 g or more and maternal weight gain (1.9% in women gaining less than recommended, 3.9% in women with the recommended weight gain and 5.8% in women gaining more than recommended, P<.01).
In this study, we examined the association of maternal weight gain on infant birth weight in four BMI groups, and we tested the IOM recommendations in this population of Danish pregnant women. We found that maternal weight gain strongly affected infant birth weight in underweight and normal weight women but less so in overweight and obese women. These latter two BMI groups were, however, quite small. The relationship between maternal weight gain and birth weight seemed to differ quite a lot in the four BMI groups. In underweight women a large weight gain led to a reduced rate of birth weight less than 3,000 g, but to an increased rate of birth weight of 4,000 g or more, whereas normal weight women, regarding infant birth weight, did not seem to benefit from a weight gain above 11–15 kg, as the rate of birth weight less than 3,000 g stalled. The association between maternal weight gain and birth weight was not significant in overweight and obese women. Our results suggest that it may be relevant to introduce recommendations for maternal weight gain in Denmark, and it is important that those recommendations be stratified according to maternal prepregnancy BMI.
Our results show that although a weight gain above recommendations in underweight women could possibly be advantageous for infant birth weight by decreasing the risk of birth weight less than 3,000 g, a large weight gain in normal weight women seems to increase the risk of birth weight of 4,000 g or more only. Our results, therefore, suggest that the upper limit for maternal weight gain in underweight women could possibly be higher, when considering optimal birth weight, whereas normal weight women should pay attention to their weight gain, which should not exceed the recommended 11.5–16 kg. Our results also suggest that overweight women should gain at least 7 kg but not more than the recommended 11.5 kg, because this would increase the risk of having a high birth weight infant without decreasing the risk of birth weight less than 3,000 g. Studies have shown that a large weight gain increases the risk of delivery complications, such as caesarean delivery and shoulder dystocia, even in the absence of macrosomia.7 This could in part be a result of an increased amount of pelvic soft tissue in obese women and women with a large weight gain.14 The association between weight gain and infant birth weight was less apparent in women with a prepregnancy BMI more than 29.0 kg/m.2 There was a similar relation between weight gain and birth weight less than 2,500 g and 4,500 g or more, but unfortunately our sample size was too small to make any conclusions regarding the risk of very low birth weight and macrosomia.
Quite alarmingly, we found that nearly 34% of underweight women in this population gained less than the recommended 12.5 kg, 39% of the obese women gained less than 7 kg, and almost 10% of the obese women actually lost weight. Nearly 40% of normal weight women gained more than the recommended 16 kg and one in 10 gained more than 20 kg. One may infer that prenatal care in Scandinavia faces a great challenge. At least one observational study has shown that it is possible to limit maternal weight gain to the recommended range if women are properly advised about physical activity and dietary habits from their physicians or other health care professionals during pregnancy.15 This same study also showed that many pregnant women receive either no or inadequate advice from health care professionals concerning pregnancy weight gain. In our study, quitting smoking during pregnancy was a significant risk factor for high weight gain, whereas women with large alcohol consumption during pregnancy had a significantly lower average weight gain.
From a population without pregestational diabetes, we selected singleton pregnancies with an outcome after 37 weeks of gestation. In Denmark, more than 94% of women who deliver are white.16 By excluding women with multiple gestations and preterm deliveries, we created an even more homogenous group, with a similar basic profile as to infant birth weight. All women in this study were included early in the second trimester of their pregnancy. Information on gestational diabetes, unfortunately, was not available in this population. Although preeclampsia entails an increased risk of low birth weight, gestational diabetes involves an increased risk of macrosomia. Both preeclampsia and gestational diabetes are more frequent in overweight and obese pregnant women.17–19
The weight gain in this study is based on information at 37 weeks of gestation, ie, for women delivering 2–3 weeks later total weight gain in pregnancy could be higher. However, using the same gestational week renders results more comparable for all women in the study. Most studies on maternal prepregnancy weight and maternal weight gain are based on self-reported weight. It has been shown that there is approximately a 1-kg difference in self-reported weight and weight registered at clinical visits.20
In conclusion, attention should be paid not only to prepregnancy BMI but also to maternal weight gain. For many years, focus on maternal weight gain has been on sufficient weight gain in underweight women and not too high a weight gain in overweight and obese women. It is, however, important to broaden the focus on optimal weight gain to include normal weight women and make sure that these women gain sufficient but not too much weight during pregnancy. In this population of Danish pregnant women, it seems that the recommendations could be applicable for normal weight women. Our results suggest that up to 40% of normal weight Danish women already gain more than 16 kg, and 10% gain more than 20 kg, a proportion that may be even higher because our results are based on weight gain at 37 weeks of gestation. It may therefore be important to limit weight gain in pregnancy to reduce the risk of complications during pregnancy and delivery—but also to avoid overweight and obesity later in life and the possible accompanying health-related problems.6,21 From the aspect of infant birth weight, the IOM recommendations may require slight modifications before being applicable in a Danish population, because the upper limit for maternal weight gain in underweight women could possibly be increased. We did, however, focus on birth weight only, and additional studies in Scandinavian populations, therefore, should be performed and should include information on other neonatal and maternal complications before Danish or Scandinavian guidelines can be worked out.
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© 2007 The American College of Obstetricians and Gynecologists
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