Secular trends in IUGR and its determinants

Both Canada and the United States have witnessed a modest decline in the incidence of low birth weight over the last two decades. Most or all of this decline is due to a lower incidence of IUGR, rather than of preterm birth (Ng and Wilkins, 1994; Kessel et al, 1984; Division of Nutrition, National Center for Chronic Disease Prevention and Health Promotion, 1994). In Canada, the birth weight of term infants increased approximately 130-140 grams between 1972 and 1986, with no apparent change in the gestational age distribution or in the growth in infants delivered preterm (Arbuckle and Sherman, 1989). Thus in both the United States and Canada, the only important change in fetal growth over the last several decades has been an increase in the size of term infants.

Based on simultaneous trends in etiologic determinants, this increase in the size of term births appears to reflect three primary factors: (1) a secular trend toward increasing prepregnancy body mass index (Flegal et al, 1988), reflecting a general trend toward increasing adiposity (which has been particularly evident in North America, but observed in most western countries); (2) a marked secular trend toward higher gestational weight gains, which have increased from 9 or 10 kilograms in the 1950's and 60's to 15 kilograms, on average, in recent years (Taffel; Eastman and Jackson, 1968; Niswander and Jackson, 1974; Keppel and Taffel, 1993); and (3) substantial decreases in cigarette smoking among pregnant women (Kleinman and Kopstein, 1987), presumably in response to increasing knowledge and publicity about the adverse effects of smoking on health, in general, and on pregnancy outcome, in particular.