Our data empirically support the use of adult BMI categories to recommend and monitor target weight gains for adolescent pregnancies, since GWG distributions are remarkably similar regardless of the way pre-pregnancy weight status is classified. We have shown that the recommended IOM weight gain ranges are considerably narrower than and shifted to the left of the actual distributions of GWG for adolescents. We found no evidence to support the use of specific GWG recommendations for adolescents of different race and age groups.
Our study addresses 2 limitations of previous GWG research. First, our sample involved only adolescents; therefore, inferences about adolescents’ gestational weight gain could be made based on empirical data. Second, we classified the pre-pregnancy weight status of adolescents using the appropriate sex- and age-specific BMI percentiles to avoid misclassification.29,30
Although our data are not from a nationally representative sample, we used a population-based database of all births in the Central and Finger Lakes regions of New York so as to eliminate the potential for selection bias. We have no reason to believe that adolescent mothers in New York are different from those across the nation. The racial distribution of all adolescents in the database is 74.8% white, 20.7% black, and 4.5% Other. We are unable to compare this racial distribution to national data on pregnant adolescents because of differences in race reporting. Despite this limitation, the generalizability of our findings should not be compromised by the racial distribution of our sample, since we did not find race to substantially alter the GWG distribution associated with optimal birth outcomes.
One limitation of this dataset, however, is that pre-pregnancy weight, used to calculate GWG and pre-pregnancy BMI, is often self-reported. With self-reported pre-pregnancy weight, there is a potential for misclassification.31
High correlations (r
5 0.96-0.98) were found, however, between self-report and measured pre-pregnancy weight.31,32
A second limitation of this study is that we restricted our sample to adolescents having optimal birthweight infants, but in clinical settings, birthweight is classified according to gestational age. The 1990 IOM report also based recommendations on a sample of women with optimal birthweight infants. Although it is not apparent whether the 2009 report considered birth-weight to define weight gain ranges, the lower and upper end of the 2009 ranges do not differ from the 1990 ones. Additionally, based on a nationally representative sample, the 10th
percentile for birthweights appropriate for 39-41 weeks gestation are close to 3000 and 4000 g, respectively (optimal birthweight as defined here).33
In our sample, 74.6% of adolescents with live singletons born between 39-41 weeks gestation gave birth to infants between 3000-4000 g. Likewise, although not a perfect comparison, the CDC national vital statistics system for 2008 indicates that 75% of US adolescents who gave birth at 40 or 41 weeks gestation had infants weighing between 3000-4000 g. Finally, we were unable to stratify by parity which could have confounded some of our results, because a significant difference in weight gain was observed for this variable. Nevertheless, neither the 1990 nor the 2009 IOM recommendations suggests using parity to modify target weight gain.
Our data concur with observations in the adult literature that GWG varies according to pre-pregnancy BMI.34–36
This observation does not hold, however, in the upper quartile of our GWG distributions, where we found little difference in weight gain by pre-pregnancy BMI overall. Additionally, the IOM recommendations are narrow compared to the adolescent weight gain distributions for all BMI categories, and they fall to the left side of the distribution rather than in the middle. Although this finding suggests that the upper limit of the IOM recommendations may be too restrictive for pregnant adolescents, our study did not take into account maternal outcomes such as postpartum weight retention, so we cannot conclude that adolescents could gain above the upper end of the IOM guidelines. Furthermore, these differences in the adolescent GWG distribution may reflect either a true difference between adolescents and adults or the effect of the 1990 GWG recommendations, which encouraged greater weight gain among all women and, in particular, among younger adolescents. In support of the latter explanation, the 2009 report6
cites data from the National Center for Health Statistics indicating a 31% increase in the proportion of women under 20 years of age with singleton pregnancies who gained at least 18 kg (above the upper end of weight gain guidelines for all BMI categories). Further research is needed to incorporate knowledge on maternal outcomes, such as postpartum weight retention, when setting the upper limits for the target weight gains. The latter was done for adults in the updated 2009 IOM recommendations, but adolescent-specific information is still too limited.
Based on our data, the use of adult BMI categories for pregnant adolescents does not appear to be problematic because the GWG distribution did not vary based on how BMI is determined. It is true, however, that some misclassification occurs when using adult BMI categories for adolescents. In our sample, among healthful-weight adolescents, 7.2% are misclassified as underweight, and 5.8% are misclassified as overweight using adult BMI cutoffs. Similarly, among overweight adolescents, 4.3% are misclassified as healthful weight, and 6.6% are misclassified as obese. Finally, 6% of obese adolescents are misclassified as overweight using the adult criteria. Thus, although there is currently no evidence to suggest that it is inappropriate to use adult BMI criteria for pregnant adolescents in clinical practice, further research should address this issue when considering maternal outcomes associated with GWG among adolescents. Making recommendations without empirical data could put those adolescents at unnecessarily higher risk for postpartum weight retention and future overweight and obesity.5,7,37,38
In conclusion, we found that the current IOM recommendations using adult BMI cut off-points to assign GWG ranges can be applied to adolescents to target and monitor weight gain in the clinical setting. With evidence indicating that excessive GWG is associated with fetal growth and obesity later in life and given the current obesity epidemic, future research should determine the best weight gain ranges that both maximizes fetal growth and minimizes postpartum weight retention.