< 18 years old, the WHO BMI cutoff points for overweight and obesity often do not correspond to the 85th and 95th percentiles, respectively, of the Centers for Disease Control and Prevention (CDC) pediatric growth charts that used to assess growth in these girls (available online at http://www.cdc.gov/nchs/data/nhanes/growthcharts/set2/chart%2016.pdf [accessed December 3, 2008]). The younger the girl, the more likely it is that she will reach the 85th or 95th percentile of the growth charts at a lower BMI value than the corresponding WHO cutoff points. Thus, if adult cutoff points are used to determine the prepregnant BMI category of younger adolescents, some girls will be categorized as being in a lighter group, leading to higher GWG recommendations than would be the case if the pediatric growth charts were used to categorize them. The committee determined that this was a tolerable risk for two reasons. First, research has shown that young teens often need to gain more weight than adult women to have an infant of the same size (Scholl, 2008). Second, it would be difficult to implement a recommendation in obstetric practices to use pediatric growth charts to categorize the prepregnant BMI of these girls.
The evidence base for women carrying multiple fetuses remains, as it was in 1990, limited. In that report (IOM, 1990), women carrying twins were encouraged to gain 16-20.5 kg (35-45 pounds) without respect to their prepregnancy BMI category. However, as discussed in Chapter 3, recent data suggest that the weight gain of women with twins who have good outcomes varies with prepregnancy BMI (see Chapter 3) as is clearly the case for women with singleton fetuses. Unfortunately, the committee was unable to conduct the same kind of analysis for women with twins as it did for women with singletons because the necessary data are unavailable. Therefore, the committee offers the following provisional guidelines, which are based specifically on the work of Luke and Hediger (Appendix C) and are corroborated by the work of others (Chapter 4):
Normal weight women should gain 17-25 kg (37-54 pounds) at term.
Overweight women should gain 14-23 kg (31-50 pounds) at term.
Obese women should gain 11-19 kg (25-42 pounds) at term.
Unfortunately, these data sources do not provide sufficient information to develop provisional guidelines for underweight women. These provisional guidelines reflect the interquartile (25th-75th percentiles) range among women who delivered their twins, who weighed ≥ 2,500 g on average, at 37-42 weeks of gestation.