relative changes in glycogen and fat exceeded the changes in amount (mg) of DNA, suggesting that a true increase in glycogen and fat per placental cell may have occurred. The increase in lipids in the placenta of the women with diabetes consisted primarily of triglycerides and phospholipids but not cholesterol (see Table C-6 in Appendix C for placental lipid content).

The optimal weight for a term infant is difficult to define. Not only are available methods for measuring fetal growth rate limited and prone to error, but fetal growth is impacted by a wide range of maternal physiological, lifestyle, and other factors. The following discussion summarizes the committee’s review of the evidence on patterns of fetal growth in singleton and multiple pregnancies and factors that alter those patterns. This information provides a foundation for understanding some of the physiological determinants of GWG identified and discussed in Chapter 4.

Singletons With the exception of longitudinal studies using methods such as ultrasound, all measures of fetal growth are cross-sectional by definition (i.e., each fetus having been measured only once) (Hytten and Chamberlain, 1991). The criteria that are commonly used are to classify fetal growth are:

• SGA (i.e., birth weight less than the 10th percentile for gestational age);

• AGA (i.e., birth weight between the 10th and 90th percentile for gestational age); and

• LGA (i.e., birth weight greater than the 90th percentile for gestational age).

These criteria were arbitrarily chosen to help assess the neonatal risk for both short-term and, more recently, long-term morbidity. Since that time there have been numerous other publications relative to fetal growth rates.

For the fetus that is deemed viable, fetal weight, as a measure of fetal growth, is usually determined at the time of delivery. The gestational age of viability has decreased steadily over the years, and the fetus is now considered potentially viable at 23-24 weeks. Therefore, most of the fetal growth curves relating to viable fetuses rely on clinical data starting from the mid-second trimester. Although the numbers are small, there appears to be minimal variation in fetal growth through 25 weeks’ gestation (Archie et al., 2006).