Gestational weight gain is associated with short- and long-term pregnancy outcomes1–10 and optimal gestational weight gain appears to vary with pregestational maternal body mass index (BMI). Body mass index is calculated as weight (kg)/[height (m)]2. Therefore, the US Institute of Medicine established gestational weight gain recommendations for women of different BMI classes in 1990,11 which were slightly revised in 2009.12 According to the current guidelines, underweight women (BMI less than 18.5) should gain 12.5-18.0 kg during their pregnancies, normal-weight women (BMI between 18.5 and 25) 11.5-16.0 kg, overweight women (BMI between 25 and 30) 7.0-11.5 kg, and obese women (BMI 30 or greater) 5.0-9.0 kg. Although these guidelines are “intended for use among women in the United States,” the Institutional of Medicine committee considered them as potentially “applicable to women in other developed countries.”12 This statement, together with the fact that analyses on the Danish National Birth Cohort seem to have had a large effect on the revision of the Institute of Medicine guidelines, makes it likely that the Institute of Medicine guidelines may be applied in other Western countries.
Although several long-term effects of gestational weight gain such as maternal weight retention or offspring's overweight have recently gained interest,13–19 optimal short-term pregnancy outcomes defined in terms of minimal risks for pregnancy complications, low birth weight, preterm delivery, stillbirths, and early neonatal deaths should constitute the benchmark for any recommendations on gestational weight gain.
The validity of the Institute of Medicine recommendations with respect to these outcomes has been assessed in several studies.4,19–22 These studies focused on comparing gestational weight gain within compared with outside the Institute of Medicine criteria. Confirming that gestational weight gain outside the Institute of Medicine recommendations is associated with poorer pregnancy outcomes, however, does not prove that adherence to these criteria is required and best suitable for achieving these outcomes. Although high or low gestational weight gain by any definition would probably be associated with an increased risk for poor pregnancy outcomes, irrespective of the cutoff values chosen, we attempted to validate the Institute of Medicine limits in comparison to population-based average gestational weight-gain ranges with respect to better or worse pregnancy outcomes.
Is adherence to the Institute of Medicine recommendations equally advisable and safe for women of all BMI classes? To address this question, we compared the risk for pregnancy outcomes by gestational weight gain with the Institute of Medicine criteria and empirically established average ranges of gestational weight gain derived from a large population-based data set comprising all singleton deliveries in Bavarian obstetric units from 2000 to 2007.
MATERIALS AND METHODS
Overall, maternal and neonatal data on 819,905 singleton deliveries in Bavarian obstetric units from 2000 to 2007 were available, including anthropometric measurements of the women in early pregnancy and before delivery as well as of the newborns. The data were extracted from a standard data set regularly collected electronically for national benchmarking of obstetric units in terms of clinical performance. The Bayerische Arbeitsgemeinschaft für Qualitätssicherung (Bavarian Working Group on Clinical Quality Assessment) conducts corresponding regional evaluations for the “Bundesland of Bavaria.” Data are transferred electronically to the Bayerische Arbeitsgemeinschaft für Qualitätssicherung office after personally identifying characteristics have been removed and replaced by an anonymous unique reference number. The data set analyzed included data of 275,708 term singleton deliveries, which we had analyzed in a previous study.1
Information on maternal weight and gestational age at the first prenatal care visit as well as information on smoking habits was abstracted by midwives and nurses from the mother's pregnancy booklet (which is issued to every pregnant woman in Germany for complete documentation of antenatal care visits) and augmented with additional information on hospital admission. Median date of first visit was the eighth completed week of gestation. Less than 1% of the recorded mothers booked before the fifth gestational week. Neonatal anthropometric data were abstracted from the hospital records. Gestational age was calculated from expected date of delivery based on date of last menstrual period and corrected by crown rump length as measured by first-trimester ultrasonography if both estimates differed considerably. Mothers were weighed on calibrated scales at each prenatal care visit. The mother's weight at the last visit, shortly before delivery, was recorded in the Bayerische Arbeitsgemeinschaft für Qualitätssicherung data. Body mass index at the beginning of pregnancy was assessed by the mother's height and weight recorded at the first visit and used to define subgroups of underweight, normal-weight, overweight, and obese mothers in accordance with the Institute of Medicine publication.12 Apart from neonatal deaths in the first 7 days of life, the database contained only information about outcomes recorded between the first visit and hospital discharge after delivery. Following an earlier study on this topic,1 we calculated gestational weight gain as last weight before delivery minus maternal weight at the first visit (both measured to the nearest kilograms) for first visits before the 14th gestational week. We therefore excluded 108,309 mothers (13.2%) whose date of the first visit was after the 13th week of gestation or unknown. Further exclusions pertained to 33,036 mothers (4.0%) with missing, extreme, or implausible values of maternal height (less than 100 cm or more than 220 cm) or weight at the first antenatal care visit or before delivery (less than 30 kg or more than 300 kg), leaving a final data set of 678,560 deliveries for our analyses. We compared the final data set with the original data set (without exclusions) with respect to mean maternal BMI at the first visit, mean gestational weight gain, gestational age, and maternal age by t tests.
We considered the following adverse pregnancy outcomes, which are routinely collected for the purpose of quality control: preeclampsia, gestational diabetes, nonelective cesarean delivery, small- and large-for-gestational-age births, preterm delivery (before 37+0 completed weeks of gestation, irrespective of underlying cause), stillbirths, and early neonatal deaths (within the first 7 days of life). Small for gestational age and large for gestational age were defined in terms of birth weight below or above the respective national 10th or 90th birth weight percentile.23 There were no missing values with respect to these outcome variables in the data set.
We calculated the prevalence of these outcomes within the gestational weight-gain ranges recommended by the Institute of Medicine (as mentioned in the Introduction) as well as in the interquartile ranges (25th to 75th percentile) and interdecile ranges (10th to 90th percentile) of gestational weight gain in each BMI category (underweight, normal weight, overweight, and obese) as observed in our data. The interquartile range and interdecile range reflect the gestational weight gain values of the average 50% and 80% of the population, respectively, and can therefore be interpreted as measures of “average” gestational weight gain at a population level. Differences in the prevalence of a specific outcome in the Institute of Medicine range compared with in the interquartile range or interdecile range were formally compared by tests on binomial proportions.
To preclude potential effects of reverse causation resulting from confounding by length of gestation, we performed a sensitivity analysis in which we calculated interquartile ranges and interdecile ranges of gestational weight gain per week after the first trimester (calculated as gestational weight gain divided by gestational age at birth minus 13 weeks) and compared the outcome prevalence with those in the respective Institute of Medicine recommendations for gestational weight gain per week in the second and third trimesters. In a sensitivity analysis, we excluded all mothers with pregestational or gestational diabetes, because these might represent a specific subgroup with special needs.
Statistical analyses were performed with SAS 9.1 and R 2.6.2.
Maternal and neonatal characteristics of the participants analyzed are shown in Table 1. Approximately 2% of the pregnant mothers had pregestational or gestational diabetes. The data analyzed did not differ considerably from the full data set (n=819,905) with respect to mean gestational weight gain, gestational age, or maternal age (data not shown). Mean BMI at the first visit was slightly higher in the original data set (25.1 compared with 24.0, P<.001).
Table 2 shows the interquartile ranges and interdecile ranges in respective BMI subgroups. Compared with the gestational weight-gain ranges recommended by the Institute of Medicine, the interquartile ranges of total gestational weight gain were relatively similar in underweight and normal-weight mothers but had considerably higher upper limits in overweight and obese women. In the latter subgroups, the interquartile ranges of gestational weight gain per week were almost completely above the respective Institute of Medicine recommendations. The interdecile ranges of both total gestational weight gain and gestational weight gain per week were substantially wider than the respective Institute of Medicine ranges in any BMI subgroup.
In underweight mothers, gestational weight gain in accordance with Institute of Medicine criteria was associated with significantly fewer preterm deliveries and small-for-gestational-age births (Table 3). Similarly, normal-weight women with gestational weight gain within the Institute of Medicine range had lower proportions of adverse pregnancy outcomes compared with gestational weight gain within the interdecile range (Table 4).
In overweight and obese mothers, the proportions of preeclampsia and nonelective cesarean deliveries were lower for gestational weight gain within the Institute of Medicine recommendations. In contrast, gestational weight gain within the Institute of Medicine ranges was associated with higher risks for gestational diabetes, small-for-gestational-age births, preterm delivery, and perinatal mortality compared with the population average as defined by interquartile ranges or even interdecile ranges (Tables 5 and 6).
In all BMI subgroups, a reduction in the proportion of small-for-gestational-age births was associated with an increasing number or large-for-gestational-age births and vice versa.
Very similar findings to those reported above were observed in both under- and overweight women if gestational weight gain per week after the first trimester was used in supplementary analyses (data not shown). In normal-weight mothers, the potential advantages of gaining weight within Institute of Medicine recommendations were less pronounced compared with the interdecile range, but the prevalence of preterm delivery was still increased in both interquartile range and interdecile range. In obese women, the prevalences of gestational diabetes and small for gestational age were similarly increased in pregnancies with gestational weight gain per week in accordance with the Institute of Medicine, whereas the prevalences of preterm deliveries and early mortality were not. The sensitivity analyses without diabetic mothers yielded similar results compared with the main analyses (data not shown).
Based on analyses of representative data on more than 650,000 deliveries in Bavarian hospitals, perinatal outcomes are likely to be improved if underweight and normal-weight pregnant women adapt their gestational weight gain to the Institute of Medicine criteria. In contrast, overweight and obese mothers seem to increase the risks for adverse perinatal outcomes related to the offspring such as small for gestational age, preterm delivery, and perinatal mortality if they restrict their gestational weight gain to the ranges recommended by the Institute of Medicine. Exclusion of mothers with pregestational or gestational diabetes did not change our main results, indicating that confounding by these factors was no major issue in our analyses.
Low and high gestational weight gain values are associated with increased risks for small for gestational age and large for gestational age, respectively.1,4–7,9 This explains why a reduction in the proportion of small for gestational age was associated with increased proportions of large for gestational age and vice versa in our analyses. However, small for gestational age is related to more severe outcomes than large for gestational age and therefore of potentially bigger concern.8,24 We therefore focused on small for gestational age rather than large for gestational age when interpreting the results from our study.
According to our results, children of underweight women seem to benefit if their mothers gain weight according to the Institute of Medicine criteria in absence of potential adverse effects related to their mothers' health. In normal-weight women, gained weight within the Institute of Medicine recommendations appeared advantageous both for the offspring and the mother compared with gestational weight gain within the interdecile range, supporting the gestational weight gain limits outlined in the Institute of Medicine criteria. In overweight and obese pregnant women, however, the picture was different. For these, a gestational weight gain within the Institute of Medicine range was associated with a lower risk for preeclampsia and nonelective cesarean delivery but with worse pregnancy outcomes related to the offspring such as preterm delivery or even perinatal mortality. Because delivery of a healthy child is the ultimate pregnancy aim, the latter outcomes appear of greater importance, leading us to the conclusion that the Institute of Medicine criteria might not be “optimal” for overweight and obese women.
The Institute of Medicine criteria have been established on the basis of a variety of observational studies with partly differing results and there is a lack of randomized trials to draw causal inferences on the associations of gestational weight gain with other pregnancy outcomes.12 Some publications have questioned the justification of the Institute of Medicine recommendations. Oken et al,8,17 for example, argued that women might also gain less weight without harmful effects. Similar suggestions were also made in commentaries.25,26 Interestingly, however, two studies showed that the risk for preterm delivery was lowest if women gained slightly above the Institute of Medicine recommendations.2,10 These findings are in accordance with those from our study for overweight and obese women, indicating that their gestational weight gain should not be limited too strictly.
It might be argued that the observed effects of gestational weight gain on preterm delivery and related outcomes might partly be explained by reverse causation. Women who deliver earlier are more likely to gain less weight because their pregnancies are shorter in duration. Our supplementary analyses with gestational weight gain per week suggested that reverse causation by length of gestation does not appear to be a major issue in our study, except for the nonincreased risk of premature delivery in obese women with gestational weight gain per week within Institute of Medicine recommendations, whereas adverse effects on small-for-gestational-age births were similarly observed in this subgroup.
It is also unlikely that our findings have been confounded by other factors that might contribute to adverse pregnancy outcomes. The prevalences of smoking in pregnancy, multiparity, severe disease in the past, substandard use of prenatal care, prior preterm delivery, and diabetes mellitus (pregestational) were very similar in the Institute of Medicine range compared with the interquartile range and interdecile range (data not shown).
Although most German obstetricians should be aware of the Institute of Medicine criteria, these are not established in the counseling of pregnant women in Germany, and therefore gestational weight gain as observed in our study is rather likely to reflect the natural course. Because both Bavaria itself and Germany as a whole can be seen as typical industrialized countries, our results should therefore be generalizable with respect to other high-income countries such as the United States. For example, comparably high mean BMI values for women have been found in Germany27 and the United States,28 and European data (eg, from the Danish National Birth Cohort) were considered in the amendment of the new Institute of Medicine guidelines.7
A major strength of the presented results is seen in the large number of pregnancies and neonates available for analysis, allowing for stratified analyses of the effect of gestational weight gain on rare adverse perinatal events. The data were collected for purposes not related to the study hypothesis. Data quality is high because completeness of the data is monitored annually across obstetric units as an integral part of benchmarking health care provision. The Bayerische Arbeitsgemeinschaft für Qualitätssicherung, providing the data for our study, is part of an established national program to generate annual statistics depicting an individual obstetric unit's deviation from national targets. This program has been run within all federal states over the last three decades. Data are collected in similar programs from the United Kingdom and the United States and have been analyzed in other studies on determinants of pregnancy outcomes.22,29–32
Some items are more stringently checked than others depending on their relevance to the process of quality assurance and their use for administrative purposes within the hospital. This explains why there were no missing values with respect to the relevant pregnancy outcomes considered in this study. Height and weight at first visit, although essential for assessing prepregnancy BMI and gestational weight gain, are not essential for either quality assurance or administrative purposes and therefore were not as rigorously checked as other variables, accounting for missing values in approximately 4% of recorded cases. Implausible values of height and weight accounted for less than 1% of these exclusions. Both variables are routinely measured by the family physician or obstetrician a long time before occurrence of the pregnancy complications considered in our survey. Therefore, missing values for height and weight at first visit are unlikely to be related to the pregnancy outcomes considered.
Selection bias might arise if the further 13% pregnancies excluded because of a late first visit were related to different interdependencies between gestational weight gain and perinatal outcomes. This, however, does not appear likely because the women in the full data set were similar to those analyzed with respect to age, duration of gestation, gestational weight gain, and BMI at the first visit. The rationale for our approach was that gestational weight gain is generally low during the first trimester but increases afterward.10,33
Previous studies on the effects of gestational weight gain on pregnancy outcomes were based on either self-reported gestational weight gain7,15,19 or the difference between weight at the first visit (in some studies at least partly self-reported4,8,9,14) and measured weight at delivery (or shortly before).3–6,8,9,14 Most women of childbearing age will not have systematic and regular weight measurements before pregnancy and self-reported weight may be influenced by recall bias. Therefore, calculating the difference between weight measurements at the first visit and before delivery performed within the medical care setting, as done in our study, appears to be the most accurate way to assess gestational weight gain.
Although our results indicate that the Institute of Medicine recommendations are not likely to be “optimal” for all mothers, it appears almost impossible to determine “optimal” gestational weight gain conclusively as long as there is no consensus on the outcomes to consider and on their relative importance.24 There may also be other factors to be considered such as smoking during pregnancy and parity.1,7 Further study is also needed for multiple pregnancies for which the Institute of Medicine recommends higher gestational weight-gain ranges than for singleton pregnancies.12
In conclusion, adopting the gestational weight gain recommendations established by the Institute of Medicine appears to be beneficial for underweight and normal-weight mothers, whereas other thresholds might be appropriate for overweight and obese mothers to avoid adverse short-term pregnancy outcomes.
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© 2010 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.
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